A twirp is a ridiculous, annoying person. You couldn't be blamed for calling your obnoxious younger brother a twirp.
The more common spelling is twerp, but either way a twirp is silly and drives you crazy. It's not nice to call someone a twirp, so you should save it for emergencies: "You twirp! Why'd you put my phone in the dishwasher?" One theory of twirp's origin says it came from the name T.W. Earp, a controversial student union president at Oxford in 1911, while other experts believe the word didn't appear until the 1920s.