The Collaborative International Dictionary of English v.0.48:

Sobriquet \So`bri`quet"\ (s[-o]`br[-e]`k[asl]"), n.[F.
   sobriquet, OF. soubzbriquet, soubriquet, a chuck under the
   chin, hence, an affront, a nickname; of uncertain origin; cf.
   It. sottobecco a chuck under the chin.]
   An assumed name; a fanciful epithet or appellation; a
   nickname. [Sometimes less correctly written soubriquet.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

sobriquet
    n 1: a familiar name for a person (often a shortened version of
         a person's given name); "Joe's mother would not use his
         nickname and always called him Joseph"; "Henry's nickname
         was Slim" [syn: nickname, moniker, cognomen,
         sobriquet, soubriquet, byname]