Sobriquet
Definition: a nickname; usually affectionate
Weirdly, the origin is unknown. It’s clearly French, but taken literally, it means “a tap (or tuck) under the chin.” No sources agreed on how we got from that to a synonym for “nickname.” It appeared in English in the 1600s.
Why I Looked It Up
From Kingmakers:
Nicknames are subsurface signatures, like watermarks. Wilson’s was the clipped and no-nonsense “A.T.,” while Sir Percy became “Kokus,” the local pronunciation of “Cox.” Writing home from Baghdad in 1917, Gertrude (Bell) took amused note of these sobriquets…