Etymology: Middle English dep, from Old English dēop; akin to Old High German tiof deep, Old English dyppan to dip — more at dip
Date: before 12th century
1. extending far from some surface or area: as
a. extending far downward <a deep well> b. (1) extending well inward from an outer surface <a deep gash> <a deep-chested animal> (2) not located superficially within the body <deep pressure receptors in muscles> c. extending well back from a surface accepted as front <a deep closet> d. extending far laterally from the center <deep borders of lace> e. occurring or located near the outer limits of the playing area <hit to deep right field> f. thrown deep <a deep pass> 2. having a specified extension in an implied direction usually downward or backward <a shelf 20 inches deep> <cars parked three-deep> 3. a. difficult to penetrate or comprehend ; recondite <deep mathematical problems> b. mysterious, obscure <a deep dark secret> c. grave in nature or effect <in deepest disgrace> d. of penetrating intellect ; wise <a deep thinker> e. intensely engrossed or immersed <she was deep in her book> f. characterized by profundity of feeling or quality <a deep sleep>; also deep-seated <deep religious beliefs> 4. a. of color high in saturation and low in lightness b. having a low musical pitch or pitch range <a deep voice> 5. a. situated well within the boundaries <a house deep in the woods> b. remote in time or space c. being below the level of consciousness <deep neuroses> d. covered, enclosed, or filled to a specified degree — usually used in combination <ankle-deep in mud> 6. large <deep discounts> 7. having many good players <a deep bull pen> Synonyms: see broad • deeply adverb • deepness noun II. adverb Date: before 12th century 1. to a great depth ; deeply <still waters run deep> 2. far on ; late <danced deep into the night> 3. a. near the outer limits of the playing area <the shortstop was playing deep> b. long 6 III. noun Date: before 12th century 1. a. a vast or immeasurable extent ; abyss b. (1) the extent of surrounding space or time (2) ocean 2. any of the deep portions of a body of water; specifically a generally long and narrow area in the ocean where the depth exceeds 3000 fathoms (5500 meters) 3. the middle or most intense part <the deep of winter> 4. any of the fathom points on a sounding line other than the marks
New Collegiate Dictionary. 2001.