Etymology
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depth (n.)

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.

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in-depth (adj.)

"profoundly, with careful attention and deep insight," 1967, from the adjective phrase (attested by 1959); see in (adv.) + depth.

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-th (2)
suffix forming nouns of action, state, or quality from verbs or adjectives (such as depth, strength, truth), from Old English -ðu, , from Proto-Germanic *-itho (cognates: Old Norse , Old High German -ida, Gothic -iþa), abstract noun suffix, from PIE *-ita (cognates: Sanskrit -tati-; Greek -tet-; Latin -tati-, as in libertatem "liberty" from liber "free"). Sometimes in English reduced to -t, especially after -h- (as in height).
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profundity (n.)

early 15c., "bottom of the sea," from Old French profundite (Modern French profondité) and directly from Late Latin profunditatem (nominative profunditas) "depth, intensity, immensity," from profundus "deep, vast" (see profound). Meaning "depth of intellect, feeling, or spiritual mystery" in English is from c. 1500.

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benthos (n.)
"life forms of the deep ocean and sea floor," 1891, coined by Haeckel from Greek benthos "depth of the sea," which is related to bathos "depth," bathys "deep, high;" which probably is Indo-European but of unknown origin. Adjective benthal is attested from 1877; benthic is attested from 1902.
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thickness (n.)
Old English þicness "density, viscosity, hardness; depth; anything thick or heavy; darkness; thicket;" see thick + -ness.
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batholith (n.)

1899, from German batholith (1892), coined by German geologist Eduard Suess from Greek bathos "depth" (see benthos) + -lith "stone."

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profoundness (n.)

early 15c., profoundnesse,  "inner part of the body;" mid-15c. as "the bottom of the sea;" late 15c. as "depth of meaning, mystery;" from profound + -ness.

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dipstick (n.)

"rod for measuring the depth of a liquid" (originally and especially the oil in a motor engine), 1927; see dip (v.) + stick (n.). For slang "penis" sense, see dip (n.2).

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intensity (n.)
1660s, from intense + -ity. Earlier was intenseness (1610s). A scientific term originally; sense of "extreme depth of feeling" attested by 1830.
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