late 14c., "to pickle, steep in vinegar," from Old French sous (adj.) "preserved in salt and vinegar," from Frankish *sultja or some other Germanic source (compare Old Saxon sultia "salt water," Old High German sulza "brine"), from Proto-Germanic *salt- (from PIE root *sal- "salt"). Related: Soused; sousing.
something steeped in pickle, especially "pig parts preserved and pickled," mid-15c., earlier "liquid for pickling" (late 14c.), from souse (v.) or from its French source.