1: of or relating to the bottom waters of the ocean depths
2: impossible to comprehend : unfathomable
Abyssal is a relatively rare word, though it's derived from the more prevalent noun, abyss. All four terms descend from the Late Latin word abyssus, which is in turn derived from the Greek abyssos ("bottomless"). Abyss and abysm are synonymous (both can refer to the mythical bottomless pit in old accounts of the universe or can be used more broadly in reference to any immeasurably deep gulf), but the adjectives abyssal and abysmal are not used identically. Abyssal can mean "incomprehensible" (as in "showed abyssal ignorance") but it's most often found in contexts referring to the bottom of the sea. Abysmal shares the oceanographic sense with abyssal, but it more frequently means "immeasurably great" or "absolutely wretched."