From Diablo Wiki
Gimp is a term that means "weakened" or "nerfed."
The word is used exclusively as a verb, as in a player complaining about a patch saying something like, "This new patch totally gimped my Wizard."
Gimped is not generally used to refer to in-game effects such as debuffs. Players would not say that a monster had gimped them with a slowing effect, as the term is generally used for external changes, usually stemming from the developers.
- See more Reference terms.