Idea  n. 1 plan etc. Formed by mental effort (an idea for a book). 2 a mental impression or concept. B vague belief or fancy (had an idea you were married). 3 intention or purpose (the idea is to make money). 4 archetype or pattern. 5 ambition or aspiration (have ideas; put ideas into a person's head).  have no idea colloq. 1 not know at all. 2 be completely incompetent. Not one's idea of colloq. Not what one regards as (not my idea of a holiday). [greek, = form, kind]