What is the X in X-mas?
early 13c., from Old French cave "a cave, vault, cellar" (12c.), from Latin cavea "hollow" (place), noun use of neuter plural of adjective cavus "hollow," from PIE root *keue- "a swelling, arch, cavity" (see cumulus). Replaced Old English eorðscrafu. First record of cave man is 1865.
early 15c., caven, "to hollow something out," from cave (n.). Modern sense "to collapse in or down" is 1707, American English, presumably from East Anglian dialectal calve "collapse, fall in," perhaps from Flemish; subsequently influenced by cave (n.). Transitive sense by 1762. Related: Caved; caving. Figurative sense of "yield to pressure" is from 1837.
A naturally occurring underground hollow or passage, especially one with an opening to the surface of the Earth. Caves can form through a variety of processes, including the dissolution of limestone by flowing water, the differential cooling of volcanic magma (which occurs when the outside surface of the lava cools, but the inside continues to flow downwards, forming a hollow tube), or the action of wind and waves along a rocky coast.
There are numerous natural caves among the limestone rocks of Syria, many of which have been artificially enlarged for various purposes. The first notice of a cave occurs in the history of Lot (Gen. 19:30). The next we read of is the cave of Machpelah (q.v.), which Abraham purchased from the sons of Heth (Gen. 25:9, 10). It was the burying-place of Sarah and of Abraham himself, also of Isaac, Rebekah, Leah, and Jacob (Gen. 49:31; 50:13). The cave of Makkedah, into which the five Amorite kings retired after their defeat by Joshua (10:16, 27). The cave of Adullam (q.v.), an immense natural cavern, where David hid himself from Saul (1 Sam. 22:1, 2). The cave of Engedi (q.v.), now called 'Ain Jidy, i.e., the "Fountain of the Kid", where David cut off the skirt of Saul's robe (24:4). Here he also found a shelter for himself and his followers to the number of 600 (23:29; 24:1). "On all sides the country is full of caverns which might serve as lurking-places for David and his men, as they do for outlaws at the present day." The cave in which Obadiah hid the prophets (1 Kings 18:4) was probably in the north, but it cannot be identified. The cave of Elijah (1 Kings 19:9), and the "cleft" of Moses on Horeb (Ex. 33:22), cannot be determined. In the time of Gideon the Israelites took refuge from the Midianites in dens and caves, such as abounded in the mountain regions of Manasseh (Judg. 6:2). Caves were frequently used as dwelling-places (Num. 24:21; Cant. 2:14; Jer. 49:16; Obad. 1:3). "The excavations at Deir Dubban, on the south side of the wady leading to Santa Hanneh, are probably the dwellings of the Horites," the ancient inhabitants of Idumea Proper. The pits or cavities in rocks were also sometimes used as prisons (Isa. 24:22; 51:14; Zech. 9:11). Those which had niches in their sides were occupied as burying-places (Ezek. 32:23; John 11:38).