abysm (n.)

"bottomless gulf, greatest depths," c. 1300, from Old French abisme "chasm, abyss, depths of ocean, Hell" (12c., Modern French abîme), from Vulgar Latin *abyssimus (source also of Spanish and Portuguese abismo), which represents perhaps a superlative of Latin abyssus or a formation on analogy of Greek-derived words in -ismus; see abyss. It survived only as a poetic variant of abyss; as late as early 17c. it was pronounced to rhyme with time.