be soft in the head—(coll.) be foolish or mentally retarded (also: be weak in the head):
- One of the curses of being a science-fiction writer is that unsophisticated people assume you to be soft in the head.
Note: The expression does not correlate in meaning with the phrase be weak-minded—be not firm or resolute enough:
- He was a good guy, but was manipulated…. I guess you could say he was weak minded.
have a hard head—be practical and determined, not influenced by sentiments:
- He’s not the suit and tie you all think he is. He has a hard head but he also has a heart.