Abbott and Costello Meet the Invisible Man
Boxer Tommy Nelson is accused of killing his manager. While detectives Bud and Lou investigate they come across an invisibility formula with which Tommy injects himself rather than face the police. This sparks an idea for trapping gangster Morgan by having Lou fight champ Rocky Hanlon, with Tommy's invisible help
There's a scene where The Invisible Man moves Abbott's spaghetti, so Abbott starts eating his napkin, like he doesn't even know that it's a napkin and not delicious food.
If you think grampa jokes like that are funny, then this movie's for you.
Spoiler: At the end, Abbott turns invisible and immediately tries to rape a bunch of nurses. So, that's weird.