An Etymological Dictionary of Astronomy and Astrophysics

1a) General: Extending well inward from an outer surface or back from an edge.

1b) Great in measure; intense. → deep exposure.

1c) Of colors, dark and vivid.

2. The deep part of a body of water, especially an area of the ocean floor having a depth greater than 5400 meters (Dictionary.com).

Etymology (EN): O.E. deop, from P.Gmc. *deupaz, from PIE *d(e)u- “deep, hollow.”

Etymology (PE): Žarf “deep;” variants Gilaki jalf, julf, jal; Tabari jol, jal, jul; Baluci jahl, johl; Kermâni jarr “deep;” Mid.Pers. zufr; Av. jafra- “deep.”
Gowd, probably ultimately from PIE root *g^wādh- “to sink, submerge;” cf. Av. vigāθô- “ravines, gorges;”
Skt. gādha- “depth; shallow;” Gk. bessa “gorge, ravine.”