1620s, from adequate + -ly (2); originally a term in logic in reference to correspondence of ideas and objects. Meaning "suitably" is recorded from 1680s.
1610s, from Latin adaequatus "equalized," past participle of adaequare "to make equal to," from ad- "to" (see ad-) + aequare "make level," from aequus (see equal). The sense is of being "equal to what is required." Related: Adequateness.