-th 1

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.

late 14c., "a deep place, deep water, the sea," also "distance or extension from the top down (opposed to height) or from without inward," apparently formed in Middle English on model of long/length, broad/breadth; from dēp "deep" (see deep (adj.)) + -th (2). Replaced older deopnes "deepness." Though the word is not recorded in Old English, the formation was in Proto-Germanic, *deupitho-, and corresponds to Old Saxon diupitha, Dutch diepte, Old Norse dypð, Gothic diupiþa.

From c. 1400 as "the part of anything most remote from the boundary or outer limit." From 1520s as "quality of extending a considerable distance downward or inward." Figurative use in reference to thought, ideas, etc., "profoundness," from 1580s.