1610s, "insufficient," from French incompétent, from Late Latin incompetentem (nominative incompetens) "insufficient," from in- "not" (see in- (1)) + Latin competentem (see competent). Sense of "lacking qualification or ability" first recorded 1630s. The noun meaning "incompetent person" is from 1866. Related: Incompetently.
1660s, "inadequacy;" 1716, "want of skill," from or modeled on French incompétence (16c.), from in- "not, opposite of, without" (see in- (1)) + compétence (see competence). Native formation incompetency is older (1610s).
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Harper, D. (n.d.). Etymology of incompetency. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/incompetency