Of uncertain derivation; a hole in the ground (dug for obtaining or holding water or other purposes), i.e. A cistern or well; figuratively, an abyss (as a prison) -- well, pit.
KJV: And he opened the bottomless pit; and there arose a smoke out of the pit, as the smoke of a great furnace; and the sun and the air were darkened by reason of the smoke of the pit.
NASB: He opened the bottomless pit, and smoke went up out of the pit, like the smoke of a great furnace; and the sun and the air were darkened by the smoke of the pit.