"condition of being proportionate or sufficient; a sufficiency for a particular purpose," 1794; see adequate + abstract noun suffix -cy. Adequateness is from 1670s.
Entries linking to adequacy
adequate adj.
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.
-cy
abstract noun suffix of quality or rank, from Latin -cia, -tia, from Greek -kia, -tia, from abstract ending -ia (see -ia) + stem ending -c- or -t-. The native correspondents are -ship, -hood.