From derivative of phthino (to wane; akin to the base of phtheiro) and opora (meaning late autumn); autumnal (as stripped of leaves) -- whose fruit withereth.
KJV: These are spots in your feasts of charity, when they feast with you, feeding themselves without fear: clouds they are without water, carried about of winds; trees whose fruit withereth, without fruit, twice dead, plucked up by the roots;
NASB: These are the men who are hidden reefs in your love feasts when they feast with you without fear, caring for themselves; clouds without water, carried along by winds; autumn trees without fruit, doubly dead, uprooted;