Someone who's wimpy is extremely ineffective, weak, or fearful. You might feel wimpy about declining an invitation to jump on your friend's snowmobile without a helmet, but that's actually a wise decision.
This informal adjective is a derogatory way to describe a cowardly or physically frail person. "Don't be such a wimpy kid," your older sister might say, "go ahead and jump off the high dive!" Most experts trace wimpy back to the comic strip character J. Wellington Wimpy. Known simply as Wimpy, he was Popeye's hamburger-loving, soft-spoken, and slightly cowardly sidekick.