As a Noun:
1. Behaviour which is not at all sensible. English = stupidity.
2. The quality of being stupid. English = stupidity.
3. The quality of being very stupid. English = idiocy.
As a Verb or Adjective:
1. To show a lack of good judgement or intelligence and to be not at all sensible. English = (be) stupid, (be) foolish.
2. To be very stupid. English = (be) idiotic.
1. Used alone to mean that you think that you do not like something and that it annoys you. English = 'How stupid!', 'That's ridiculous!'
2. Used alone to express irritation with yourself for having done something that is foolish or a mistake. English = 'Oops!'