abscess (n.)

in pathology, "collection of pus in some part of the body," 1610s, from Latin abscessus "an abscess" (the Latin word was used in a medical sense by Celsus), literally "a going away, departure," from the stem of abscedere "withdraw, depart, retire," from ab "off, away from" (see ab-) + cedere "to go, withdraw" (from PIE root *ked- "to go, yield"). The notion is that humors "go from" the body through the pus in the swelling.