Old English gyltig "offending, delinquent, criminal," from gylt (see guilt (n.)). In law, "that has committed some specified offense," late 13c. Of conscience, feelings, etc., 1590s. Meaning "person who is guilty" is from 1540s. To plead not guilty is from 15c.; to plead guilty is 19c., though, as OED notes, "Guilty is technically not a plea, but a confession." Related: Guiltily; guiltiness.