Old English 'deop' (deep) + abstract suffix '-th' — from PIE *dhewb- (hollow). To be deep is to be hollowed out.
The distance from the top or surface of something to its bottom; the quality of being deep.
From Middle English depthe, formed from Old English dēop (deep) with the abstract suffix -th (as in width, length, strength, warmth, filth, growth). Old English dēop (deep, profound, intense) derives from Proto-Germanic *deupaz (deep, low-lying), from PIE *dhewb- (deep, hollow, deep-bottomed vessel). The PIE root *dhewb- connects depth etymologically to hollowness: to be deep is to be hollowed out beneath, like a bowl
The suffix '-th' in 'depth' is the same ancient suffix found in 'width' (wide + -th), 'length' (long + -th), 'strength' (strong + -th), 'growth' (grow + -th), and 'health' (whole/hale + -th). It turns adjectives and verbs into abstract nouns. It is one of the oldest suffixes in Germanic, inherited from Proto-Indo-European, yet no new words