Other forms: geezers
A geezer is an old person. It might hurt your grandfather's feelings if you refer to him as a geezer, since he thinks of himself as being youthful.
Use the word geezer when you need an informal, slightly rude word for "wacky old man." If you're in the UK, you can call any man, whatever age he is, a geezer, but in the US it's specifically an old guy. The root of geezer is the now-obsolete Cockney word guiser, which means "mummer," or "masked mime or actor.