A character whose lithe, lightweight appearance suggests a Fragile Speedster (or, at the least, a character of average size compared to others) but it turns out they are a Mighty Glacier.
Compare with Cute Bruiser, where a fragile-looking character is strong, but not slow (also, this trope is not confined to female characters). Often feature Muscles Are Meaningless. This trope is the inversion of Acrofatic. May be a Super-Strong Child depending on age.
Not to be confused with an Ice Queen, which has more to do with personality than toughness.
Asuna of Mahou Sensei Negima! has a relatively average size in the cast (taller than most, but far leaner than the other warriors). Despite that, she carries a BFS, wears thick armor, and is no where near as fast as the other warriors in the cast.
Flandre of Princess Resurrection is easily one of the shortest in the cast (she appears as a young girl). Being a gynoid, she's incredibly strong as well as quite slow due to her heavy weight.
Claudette from Queen's Blade qualifies since she is thin like the rest of the cast but she feels no need to use much speed in battle when she can simply summon lighting strikes on her enemies.
Double subverted with Chelsea. She is fairly nippy while running, hopping, and skipping about the arena, but her attack speed is slow compared to the likes of Stahn (and justifiably so, since she's the archer). Even so, her techs still cause some respectable damage when they hit home.
From the same game, Philia is a magical version of this trope played straight, probably due to her being weighed down by her BFS. So what if she can't dance around the air like Rutee and Woodrow? She can cast Meteor Storm!
Hinako Shijo in King of Fighters looks much like the rest of the female cast (she's thin, short and only 16 years old). Despite that, she's one of the slower characters who uses strength to her advantage. Why? Her style is sumo wrestling.
The rather petite Ashlotte from Soul Calibur IV is a reskin of Mighty GlacierAstaroth (as is obvious from her name). She shares his moveset, strength and low agility despite being barely more than half his size. She entirely made of an unknown metal alloy, powered by dark magic, and is the second heaviest character in the game (only Angol Fear, who's an Eldritch Abomination of sorts and thus shouldn't be judged based on looks alone, is heavier), so her hitting like a dump truck and being about as fast in spite of being tiny compared to Astaroth makes sense - all that steel can't possibly move fast, and with so much mass behind her blows, getting hit by her is roughly equivalent to getting run over by a car.
In games with build customization like City of Heroes and Phantasy Star Universe, build is often purely aesthetic, so as long as your weak-looking character is of the correct class, they can deal out more punishment while moving more slowly than stronger-looking characters that went with another class.
Ike can't exactly be called a "waif", but in Super Smash Bros. Brawl, he's a pipsqueak when compared to other Mighty Glaciers like Bowser, Ganondorf and Donkey Kong. Despite this, he's a heavy character, attacks slowly, moves slowly, and hits like a train (the developers actually went so far as to update his design by making him look buffer in the next game to reflect later FE installments). In the Fire Emblem series, Ike is actually more of a Lightning Bruiser.
Mr. Game and Watch looks like a lightweight and is knocked back like one, but its moves are on the "strong and slow" side.
Super Mario Bros.: Spin-offs after the release of Mario Kart Wii classify Rosalina and Waluigi in the same rank as the massive heavyweights despite their thin figures; this is because Rosalina is a Statuesque Stunner, and Waluigi is Lean and Mean. In Rosalina's case, it extends beyond spin-offs, as her playable appearance in Super Mario 3D World makes her the slowest character in the team, but the only one capable of attacking enemies without power ups.
Glaceon is like this, with amazing Special Attack and low Speed.
Scizor looks like a Fragile Speedster as it's rather scrawny and seems capable of moving at high speeds. It even learns moves like Bullet Punch and Agility. Its highest stats are actually Attack and Defense with well below average Speed. Scizor's lasting power in spite of its Speed makes it popular to use.
Azumarill. A cute Aqua Rabbit Pokemon a bit under 3 feet tall, with decent defenses, rubbish speed, and an Ability that doubles its Attack to make it the 11th most powerful physical attacker.
In The Elder Scrolls V: Skyrim, create a female Breton of the smallest weight possible and load up on heavy armor and two-handed weapons. Since Bretons are the physically smallest of all of the races you can play as, and female Bretons are even smaller, and you're roaming Skyrim, the land of the enormous, muscular Nords, you're essentially playing a tiny, slow-moving armor-plated waif of death whose spiked helmet barely comes up to most people's shoulders.
Fuuka Kazamatsuri of Disgaea 4 is a skinny middle-school girl with no demonic lineage or combat training. This does not stop her from being the resident Mighty Glacier.
Ness from EarthBound has the build of an ordinary 13-year-old boy, the highest offense out of all the party members, and is quite slow.
Audrey Heitinga from Valkyria Chronicles, a 17-year old Lancer in Squad 7. Despite how well-armored, heavily built, and low-mobility her class is known for being, she's quite slender yet can crush a watermelon with her bare hands. She also has the highest potential for anti-tank damage in the game, meaning at her peak she could theoretically tear through the Batomys in a single shot.
Raquel Applegate from Wild ARMs 4 is a small to average sized young woman with a BFS. She is slow enough that the other party members often get 2-3 turns for each of her turns, and hits hard enough to, with the right setup, defeat even Ragu O Ragula in a single blow.
Heart Girl from the Global Guardians PBEM Universe. Imagine an 8 year old girl with Superman's strength and invulnerability. And nothing else. She moves and reacts about as quickly as you'd expect for an awkward pre-pubescent girl who isn't all that athletic or coordinated. But she punches like a mountain.
Toph Beifong from Avatar: The Last Airbender, being the The Big Guy of the Gaang despite being youngest, smallest and blind. She's actually an earthbender, and a powerful one. Before she joined the Gaang, she was participating in the underground local-type-wrestling league, fighting against bigger and tougher people. In the end of her introductory episode, she sweeps the floor with six of them at once. She doesn't fit the "glacier" part of the definition until she invents metalbending, which allows her to bend metal into an impromptu suit of armor at the cost of movement speed.