1590s, from French adhérer "to stick, adhere" (15c., corrected from earlier aderer, 14c.) or directly from Latin adhaerare "to stick, cling to," from ad "to" (see ad-) + haerere "to stick" (see hesitation). Originally often of persons, "to cleave to a leader, cause, party, etc." (compare adherent (n.), which still often retains this sense). Related: Adhered; adhering.