Yours, Etc.: Origins and Uses of 8 Sign-Offs
past participle suffix of weak verbs, from Old English -ed, -ad. --od (leveled to -ed in Middle English), from Proto-Germanic *-do- (cf. Old High German -ta, German -t, Old Norse -þa, Gothic -da, -þs), from PIE *-to- (cf. Sanskrit -tah, Greek -tos, Latin -tus).
Originally fully pronounced, as still in beloved (which, with blessed, accursed, and a few others retains the full pronunciation through liturgical readings). In 16c.-18c. often written -t when so pronounced (usually after a consonant or short vowel), and still so where a long vowel in the stem is short in the pp. (e.g. crept, slept, etc.). In some older words both forms exist, with different shades of meaning, e.g. gilded/gilt, burned/burnt.