Origins
The word 'depth' is formed from the adjective 'deep' (Old English 'dΔop') plus the abstract noun suffix '-th' (Old English '-ΓΎu'), a formation pattern that dates to Proto-Germanic.βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ The same suffix appears in 'width' (wide + -th), 'length' (long + -th), 'breadth' (broad + -th), 'strength' (strong + -th), 'growth' (grow + -th), 'health' (hale/whole + -th), 'stealth' (steal + -th), 'warmth' (warm + -th), 'youth' (young + -th), and 'truth' (true + -th). This suffix is one of the oldest grammatical tools in Germanic, inherited from PIE, but it is no longer productive β no new English words are being coined with it.
The base word 'deep' descends from Old English 'dΔop' (deep, profound, extending far downward), from Proto-Germanic *deupaz (deep, hollow), from PIE *dhewb- (deep, hollow). The PIE root connects depth fundamentally to hollowness β to be deep is not merely to extend downward but to be hollowed out, to contain a void. This semantic link between depth and emptiness persists in expressions like 'the depths of despair' and 'in the depth of winter,' where depth implies an interior void or an extreme low point.
The Germanic cognates are consistent: German 'tief' (deep), Dutch 'diep' (deep), Old Norse 'djΓΊpr' (deep), Gothic 'diups' (deep), Swedish 'djup' (deep). The initial consonant shift from PIE *dh- to Germanic *d- is regular (Grimm's Law), and the vowel alternation follows the normal ablaut patterns. The word 'dip' (to plunge briefly into liquid) may belong to the same family, from a causative or iterative formation meaning 'to make deep' or 'to go deep repeatedly.' The connection to 'dive' (to plunge headfirst into water) is also possible but less certain etymologically.
Old English Period
The abstract noun 'depth' itself first appears in Middle English around 1350, replacing the earlier Old English forms 'dΔopnes' (deepness) and 'dΘ³pΓΎ.' The '-ness' and '-th' suffixes competed for centuries as means of forming abstract nouns from adjectives. In some cases both survive with differentiated meanings: 'depth' and 'deepness' coexist in Modern English, but 'depth' has acquired more concrete and technical uses (the depth of a pool, depth perception, depth charge), while 'deepness' remains purely abstract (the deepness of his voice).
In nautical usage, depth has been measured for millennia. The 'fathom' (from Old English 'fæþm,' the span of outstretched arms, about six feet) was the traditional unit of water depth, measured by lowering a weighted 'sounding line' and counting the arm-spans of rope paid out. The phrase 'to fathom' (to understand) comes from this practice β to fathom a problem is to sound its depth, to measure how deep it goes. 'Unfathomable' means both 'too deep to measure' and 'too profound to understand.'
The metaphorical extension of 'depth' to intellectual and emotional domains is ancient and cross-linguistic. 'Depth of thought,' 'depth of feeling,' 'in-depth analysis,' and 'depth of character' all use the physical image of downward extension to describe intensity, thoroughness, or profundity. The metaphor is spatial: understanding is conceived as vertical, with superficial knowledge 'on the surface' and profound knowledge 'in the depths.' This DEEP IS DOWN metaphor is one of the most universal conceptual metaphors identified by cognitive linguists, appearing in virtually every language studied.