abyss (n.) Look up abyss at Dictionary.com
late 14c. in Latin form abyssus; early 14c. as abime (from a form in Old French; see abysm), from Late Latin abyssus "bottomless pit," from Greek abyssos (limne) "bottomless (pool)," from abyssos "bottomless, unfathomed," hence, generally, "enormous, unfathomable," also as a noun, he abyssos "the great depth, the bottomless pit." This is a compound of a- "without" (see a- (2)) + byssos "bottom," a word of uncertain origin possibly related to bathos "depth" [Liddell & Scott]. Watkins, however, suggests a connection with the root of bottom (n.).

The current form is a 16c. partial re-Latinization. OED notes, "the word has had five variants, abime, abysm, abysmus, abyssus, abyss; of which abyss remains as the ordinary form, and abysm as archaic or poetic." In reference to a seemingly bottomless gulf from 1630s.