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; pustular.
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