As a Verb or Adjective:
1. To be of very bad quality and not to be liked at all; to be worth nothing at all. English = (be) lousy, (be) rotten; (be) worthless.
2. To severely harm, damage or spoil something; to damage something so that it no longer works. English = ruin.
3. (When talking of food, wood or the bodies of animals) to become no good because they are getting old, dying, or are dead and are being attacked by micro-organisms or insects. English = rot, decay, go off.
1. Used alone to mean you think something is of very bad quality or that you think someone has very poor skills or abilities. English = 'Hopeless!', 'What crap!' and so on.