early 15c., "the scourging of Christ," from Old French flagellacion "scourging, flogging," or directly from Latin flagellationem (nominative flagellatio) "a scourging," noun of action from past-participle stem of flagellare "to scourge, lash" (see flagellum). In a general sense from 1520s.
flaccid
flack
flag
flagellant
flagellate
flagellation
flagellum
flageolet
flagitious
flagman
flagon