| mentally defective |
| mentally deficient(expression, silly, foolish, stupid) |
| mentally handicapped(expression, silly, foolish, stupid) |
| mentally retarded(expression, idiom) |
| mentally strange(expression, silly, foolish, mad, insane) |
| moonstruck(adjective, silly, foolish, stupid, mad) |
| nitwitted(adjective, simple, silly, foolish, stupid) |
| nutty as fruitcake(expression, idiom, silly, foolish, mad) |
| of unsound mind(expression, idiom, silly, stupid, mad) |
| out to lunch(expression, idiom, dull, silly, mad) |
| potty(adjective, wild, silly, foolish, stupid) |
| psychopathic(silly, foolish, stupid) |
| rash(adjective, dull, simple, slow, silly) |
| reckless(silly, thoughtless, unwise) |
| round the bend(expression, idiom, silly, foolish, mad) |
| screw loose(expression, silly, foolish, mad, insane) |
| screwball(adjective, silly, foolish, mad, insane) |
| sentimental |
| soft(adjective, silly, stupid, green, fat) |
| softheaded(adjective, knowledge, ability) |