late 14c., from Old French suplicacion "humble request," from Latin supplicationem (nominative supplicatio) "a public prayer, thanksgiving day," noun of action from past participle stem of supplicare "to beg humbly" (in Old Latin as sub vos placo, "I entreat you"), from sub "under" (see sub-) + placare "to calm, appease, quiet, soothe, assuage," causative of placere "to please" (see please). In ancient Rome, a religious solemnity, especially in thanksgiving for a victory or in times of public danger.
