As a Verb or Adjective: 1. Of someone or something, to do something very badly. English = hopeless, lousy. 2. To lack the knowledge and ability to do something well, especially something relatively simple. English = (be) hopeless, (be) lousy. 3. To behave in a silly and immature way because you are not sensible enough to behave properly. English = childish. Formal English = puerile.