The term 'KEEP IT UNDER YOUR HAT!' is an idiom that means to keep something a secret. The crossword solutions for this term are HEAD (4 letters), HAIRDO (6 letters), DONTTELL (8 letters), and MUMSTHEWORD (11 letters). This phrase can be interpreted as a warning to not reveal a secret, with the solutions representing the idea of covering one's HEAD, styling one's hair, not speaking, and keeping silent, respectively.