yellowish-white inflammatory exudation, consisting of white blood cells, etc., produced by suppuration, 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" (source also of Sanskrit puyati "rots, stinks," putih "stinking, foul, rotten;" Greek puon "discharge from a sore," pythein "to cause to rot;" Lithuanian pūvu, pūti "to rot;" Gothic fuls, Old English ful "foul"), perhaps originally echoic of a natural exclamation of disgust.
The formation of pus is called suppuration. A collection of pus within the solid tissues is called an abscess. A suppurating open sore is an ulcer. [Century Dictionary]