13 letters · 23 points in Scrabble
adjective lacking the power to produce a desired effect
originEtymology tree Proto-Indo-European *né Proto-Indo-European *n̥- Proto-Italic *n̥- Latin in-bor. Middle English in- English in- English efficacious English inefficacious From in- + efficacious.
Etymology from Wiktionary, CC BY-SA 3.0