Etymology
Advertisement
deep (n.)

Old English deop "deep water," especially the sea, from the source of deep (adj.). Cognate with Old High German tiufi, German Tief, Teufe, Dutch diep, Danish dyb. General sense of "that which is of great depth" is by mid-14c.

Related entries & more 
Advertisement
deep (adv.)

"far down, deeply," Old English deope, from the source of deep (adj.).

Related entries & more 
deep (adj.)

Old English deop "having considerable extension downward," especially as measured from the top or surface, also figuratively, "profound, awful, mysterious; serious, solemn," from Proto-Germanic *deupaz (source also of Old Saxon diop, Old Frisian diap, Dutch diep, Old High German tiof, German tief, Old Norse djupr, Danish dyb, Swedish djup, Gothic diups "deep"), from PIE root *dheub- "deep, hollow" (source also of Lithuanian dubus "deep, hollow," Old Church Slavonic duno "bottom, foundation," Welsh dwfn "deep," Old Irish domun "world," via sense development from "bottom" to "foundation" to "earth" to "world").

By early 14c. "extensive in any direction analogous to downward," as measured from the front. From late 14c. of sound, "low in pitch, grave," also of color, "intense." By c. 1200, of persons, "sagacious, of penetrating mind." From 1560s, of debt., etc., "closely involved, far advanced."

Deep pocket as figurative of wealth is from 1951. To go off the deep end "lose control of oneself" is slang recorded by 1921, probably in reference to the deep end of a swimming pool, where a person on the surface can no longer touch bottom. When 3-D films seemed destined to be the next wave and the biggest thing to hit cinema since talkies, they were known as deepies (1953)., hard to understand

Related entries & more 
knee-deep (adj.)
c. 1400, "up to the knees," from knee (n.) + deep (adj.).
Related entries & more 
deep-sea (adj.)

"of or pertaining to the deeper parts of the ocean," 1620s, from deep (adj.) + sea.

Related entries & more 
Advertisement
deep-set (adj.)

"set far downward or inward," originally of eyes, late 14c., from deep (adv.) + past participle of set (v.).

Related entries & more 
deep-seated (adj.)

1741, "having its root far below the surface," hence "firmly implanted," from deep (adv.) + past participle of seat (v.). Figurative use is by 1847. Deep-rooted is from early 15c.

Related entries & more 
deepness (n.)

Old English deopnes "deep water," also "a mystery or secret;" see deep (adj.) + -ness. From late 12c. as "distance downward;" c. 1200 as "wisdom, profundity."

Related entries & more 
deeply (adv.)

Old English deoplice "at or to a great depth," used in both literal and figurative senses; see deep (adj.) + -ly (2). Meaning "profoundly, thoroughly" is from c. 1200; that of "with strong feeling, passionately" from c. 1400. Of sighing from 1550s; of drinking from 1690s.

Related entries & more 
deepen (v.)

c. 1600, transitive, "to make deep or deeper," from deep (adj.) + -en (1). Intransitive sense of "become deep or deeper" is from 1690s. Related: Deepened; deepening. The earlier verb had been simply deep (Middle English deopen), from Old English diepan.

Related entries & more