past-participle suffix of weak verbs, from Old English -ed, -ad, -od (leveled to -ed in Middle English), from Proto-Germanic *-da- (cognates: Old High German -ta, German -t, Old Norse -þa, Gothic -da, -þs), from PIE *-to-, "suffix forming adjectives marking the accomplishment of the notion of the base" [Watkins] (cognates: Sanskrit -tah, Greek -tos, Latin -tus; see -th (1)).
Originally fully pronounced, as still in beloved (which, with blessed, accursed, and a few others retains the full pronunciation through liturgical readings). In Old English already the first and third person singular past tense form of some "weak" verbs was -te, a variant of -de (see -ed), often accompanied by a change in vowel sound (as in modern keep/kept, sleep/slept).
A tendency to shorten final consonants has left English with many past tense forms spelled in -ed but pronounced "-t" (looked, missed, etc.). In some older words both forms exist, with different shades of meaning, as in gilded/gilt, burned/burnt.