O.E. deop, from P.Gmc. *deupaz, from PIE *d(e)u- "deep, hollow" (cf. O.C.S. duno "bottom, foundation," O.Ir. domun "world," via sense development from "bottom" to "foundation" to "earth" to "world"). Figurative sense was in O.E.; extended 16c. to color, sound. Deep pocket "wealth" is from 1951. Deep-freeze was a registered trademark (U.S. Patent Office, 1941) of a type of refrigerator; used generically for "cold storage" since 1949. To go off the deep end "lose control of oneself" is slang first recorded 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). The gods have spared us.
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
in deep in the Bible
used to denote (1) the grave or the abyss (Rom. 10:7; Luke 8:31); (2) the deepest part of the sea (Ps. 69:15); (3) the chaos mentioned in Gen. 1:2; (4) the bottomless pit, hell (Rev. 9:1, 2; 11:7; 20:13).
Seriously involved; far advanced. For example, He was in deep with the other merchants and couldn't strike out on his own, or She used her credit cards for everything, and before long she was in deep.
in deep water. Also,
in over one's head. In trouble, with more difficulties than one can manage, as in The business was in deep water after the president resigned, or I'm afraid Bill got in over his head. These metaphoric expressions transfer the difficulties of being submerged to other problems. The first appears in Miles Coverdale's 1535 translation of the Book of Psalms (68:13): “I am come into deep waters.” The second, which also can signify being involved with more than one can understand, dates from the 1600s. Also see over one's head