pus (n.) Look up pus at Dictionary.com
late 14c., from Latin pus "pus, matter from a sore;" figuratively "bitterness, malice" (related to puter "rotten" and putere "to stink"), from PIE *pu- (2) "to rot, decay" (cognates: Sanskrit puyati "rots, stinks," putih "stinking, foul, rotten;" Greek puon "discharge from a sore," pythein "to cause to rot;" Lithuanian puviu "to rot;" Gothic fuls, Old English ful "foul"), perhaps originally echoic of a natural exclamation of disgust.