inadequate (adj.)

word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.

1610s, "equal to what is needed or desired, sufficient," from Latin adaequatus "equalized," past participle of adaequare "to make equal to, to level with," from ad "to" (see ad-) + aequare "make level," from aequus "equal, even" (see equal (adj.)).

The sense is of being "equal to what is required." It shares duty with enough, depending on the subject. Somewhat disparaging use, "mediocre, just good enough," is by 1900. Related: Adequateness.

"incompleteness, ineffectiveness, state or quality of being insufficient," 1764, from inadequate + abstract noun suffix -cy. Related: Inadequacies.