"to place in a tomb, bury, inter," 1570s, from Old French entomber "place in a tomb," from en- "in" (see en- (1)) + tombe "tomb" (see tomb). Related: Entombed; entombing. The earlier verb was simply tomb (c. 1300).
word-forming element meaning "in; into," from French and Old French en-, from Latin in- "in, into" (from PIE root *en "in"). Typically assimilated before -p-, -b-, -m-, -l-, and -r-. Latin in- became en- in French, Spanish, Portuguese, but remained in- in Italian.
Also used with native and imported elements to form verbs from nouns and adjectives, with a sense "put in or on" (encircle), also "cause to be, make into" (endear), and used as an intensive (enclose). Spelling variants in French that were brought over into Middle English account for parallels such as ensure/insure, and most en- words in English had at one time or another a variant in in-, and vice versa.
c. 1200, tumbe, early 14c. tomb, from Anglo-French tumbe and directly from Old French tombe "tomb, monument, tombstone" (12c.), from Late Latin tumba (also source of Italian tomba, Spanish tumba), from Greek tymbos "mound, burial mound," generally "grave, tomb."
Watkins suggests it is perhaps from PIE root *teue- "to swell," but Beekes writes that it is probably a Pre-Greek (non-IE) word. He writes that Latin tumulus "earth-hill" and Armenian t'umb "landfill, earthen wall" "may contain the same Pre-Greek/Mediterranean word," and suggests further connections to Middle Irish tomm "small hill," Middle Welsh tom "dung, mound."
The final -b began to be silent about the time of the spelling shift (compare lamb, dumb). Modern French tombeau is from Vulgar Latin diminutive *tumbellus. The Tombs, slang for "New York City prison" is recorded from 1840.