"small, inflammatory sore or tumor containing pus," late 14c., from Old French pustule (13c.) and directly from Latin pustula "blister, pimple" (from PIE imitative root *pu- (1) "blow, swell," on notion of "inflated area;" source also of Sanskrit pupphusah "lung," Greek physa "breath, blast, wind, bubble," Lithuanian pučiu, pūsti "to blow, swell," Old Church Slavonic puchati "to blow"). Compare emphysema. Related: Pustulant; pustulate; pustulation.
word-formation element meaning "pertaining to, of the nature of," from Latin -arem, -aris "of the kind of, belonging to," a secondary form (by dissimilation) of -alis, used after syllables with an -l- (such as insularis for *insulalis, stellaris for *stellalis).