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Many heroes will wear armor and weapons that are invisible to casual observation because they look like part of their normal clothing. This has many advantages, not the least of which is being able to take them places where weapons are not normally allowed.

This kind of double purpose clothes are a common hallmark of a Badass. This trope is sometimes a Subtrope of Clothes Make the Superman and Clothing Combat, but not always. It is often a Supertrope to Hat of Power. Weaponized Headgear and Sword Cane are Subtropes.

Examples:

Anime and Manga

Gunsmith Cats: Bean Bandit's outfits.

Comic Books

Lady Killer from The Strangers in The Ultraverse was a superhero who was also a fashion designer. Every part of her costume could be used as a weapon.

Film

Oddjob's razor brimmed bowler hat in Goldfinger.
Taken to the point of absurdity in Casino Royale 1967 — Q had just developed a vest with one hundred different (and painfully obvious) functions.

Literature

The Lookingglass Wars series: The entire Millenary, but Hatter Madiagan's wardrobe in particular.
Discworld:
- Adorabell Dearhart, AKA "Spike", AKA "Killer", has stiletto heals that can punch through two reinforced layers of cured leather and a human foot and still embed themselves into an oak floor board.
- Sam Vimes butler Wilkins used to own a hat with Sharpened pennies sewn into the brim for fighting; when he was a street ruffian in his youth.
- Granny Weatherwax is never seen without her pointy black hat, held on with several hatpins that she uses as weapons in the rare cases she needs a weapon at all.

Tabletop RPG

Shadowrun. The 2nd Edition supplement The Neo-Anarchists' Guide to Real Life had a section dedicated to armor and weapons disguised as clothing. They included the Barton Arms Bracer (a gun disguised as a piece of jewelry) and the Barton Arms Gun Cane (a cane that can fire a bullet). A variety of fashion houses (Armante, Mortimer of London, Vashon Island and Zoe) have formal wear that could stop a bullet.
D&D : Monks; everything they wear can be used as a weapon.

Video Games

Hades in Kid Icarus Uprising.
Viridi: He's covered from head to toe in weaponry. Like a rainbow apocalypse.
Hades: I prefer to think of it as my devastation ensemble. I only wear it for very special occasions. You know, weddings, armageddons...
In Malicious, the story revolves around a boy or girl called a Spirit Vessel, that have been charged with protecting humanity with scarfs that shapeshift into weapons called Mantle of Cinders. With that, they have to stop a great evil called '"The Malicious"', from destroying the world.
Cerebella of Skullgirls, wears a monster-hat named Vice-Versa, that acts as the muscles and Cerebella as the knives.
Several of the multiplayer personas in the Assassin's Creed disguise their weapons as attire. For example, one female has blades in her paper fan, and another stabs removes her Hair Decoration and stabs people's eyes with the metal prongs. Makes sense, because the multiplayer is assassins assassinating other assassins (and trying not to be assassinated themselves) in an assassination training simulator, so they have every reason to go covert.

Web Comics

In Schlock Mercenary, the standard uniform for Tagon's Toughs is actually nanofiber armor with a deployable breathmask and allows antigrav flight as well. The uniform that Legs uses also has a pair of cannons built into the helmet, as she lacks humanoid hands with which to hold a weapon.

Western Animation

Batman The Brave And The Bold: In "Joker: The Vile and the Villainous", the Weeper has a handkerchief that can be used as a sword and a lasso (amongst other things).
In one episode of "Adventure Time", Finn and Jake repair a shoe for a funeral atendee and include a knife in the toe.

Real Life