1712, formed irregularly from Latin acer (fem. acris) "sharp, pungent, bitter, eager, fierce," from PIE *akri- "sharp," from root *ak- "be sharp, rise (out) to a point, pierce" (cognates: Oscan akrid (ablative singular) "sharply;" Greek akis "sharp point," akros "at the farthest point, highest, outermost," akantha "thorn," akme "summit, edge;" also oxys "sharp, bitter;" Sanskrit acri- "corner, edge," acani- "point of an arrow," asrih "edge;" Lithuanian ašmuo "sharpness," akstis "sharp stick;" Old Lithuanian aštras, Lithuanian aštrus "sharp;" Old Church Slavonic ostru, Russian óstryj "sharp;" Old Irish er "high;" Welsh ochr "edge, corner, border;" Old Norse eggja "goad;" Old English ecg "sword"). The -id suffix probably is in imitation of acid. Acrious (1670s) is a correct formation, but seldom seen.