Man's best friend is a phrase often used to refer to dogs, which have been domesticated for thousands of years. The crossword solutions for this phrase are 'DOG' (3 letters) and 'FIDO' (4 letters). FIDO is a common name for a DOG, and is derived from the Latin word 'fidelis' meaning faithful or loyal. Dogs have been known to form strong bonds with their owners, and are often considered to be a man's best friend.