abysmal (adj.)

"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.

1690s, "unfathomable, unsearchably deep, like an abyss," from abyss + -al (1). Since 19c. mainly "inhabiting or belonging to the depths of the ocean" (used especially of the zone of ocean water below 300 fathoms), though in 19c. abysmal was more common in oceanography.