early 15c., "action of growing out," from Latin excrescentia (plural) "abnormal growths," from excrescentem (nominative excrescens), present participle of excrescere "grow out, grow up," from ex "out" (see ex-) + crescere "to grow" (from PIE root *ker- (2) "to grow"). Meaning "that which grows out abnormally" (on a living thing) is from 1570s (excrescency in this sense is 1540s).
