Basic Trope: An old man (or woman) who has serious butt-kicking capabilities, even if they're rarely shown.
Straight: Grandpa Bob usually settles for giving boring lectures of his Glory Days, but when the chips are down, he defends his home from twenty intruders without breaking a sweat or losing his dentures.
Grandpa Bob is always stealing the show during big fights, to the point that his grandchildren are getting embarrassed.
Zig-Zagged: Somedays Grandpa Bob is just a senile old man, others he's the most dangerous man alive.
Averted: Grandpa Bob, though a mighty fighter in his old days, has dwindled in strength and skill, and the family is sure to keep him away from any danger.
Enforced: "See this old guy? He needs to get some of the spotlight, show where The Hero gets his skill from."
"How is this guy so strong?! He's like, what, eighty?"
"Hernia acting up again, grandpa?" "...Yep. Gotta stop pickin' the punks up an' throwin' 'em..."
Invoked: Over the decades, Grandpa keeps himself in fighting shape, just because he knows a war is coming, sooner or later.
Defied: Grandpa retired from combat years ago, sick of the bloodshed. He hasn't trained a day since.
Being a badass is all nice and well but there's a price. How much time did it take for Grandpa Bob to become badass? How hard is it to stay that way as time goes on? Fame as a badass attracts challengers and that's a lot of headaches for Grandpa Bob and his grandkids over the years.
Grandpa Bob's as strong as ever, but his mental faculties aren't what they used to be, and he sometimes forgets things like where he is or what's happened over the past ten years... With ugly results, as his way of getting answers can be downright terrifying.
But hey, Grandpa Bob is badass. He can deal with it.
While dying from over-exertion, he inspires new respect from his family and is remembered by the public as a hero.