Phthinoporinos (φθινοπωρινός) means 'of or belonging to late autumn' — describing something characteristic of the season when fruit has passed and leaves are falling. From phthino (to wane/decline) + opora (late summer/harvest fruit). It appears only in Jude 12, where false teachers are called 'autumn trees without fruit' — all the outward form of a fruit tree, but at the very season when fruit is expected, they are barren.
Jude's use of phthinoporinos is scathing: these false teachers are trees in autumn — precisely when you expect fruit, there is none. Furthermore they are 'twice dead, uprooted.' The image captures the complete failure of promise: a tree in harvest season that bears nothing has failed its entire purpose. Jesus used the same principle in cursing the fig tree (Mark 11) — religious appearance without life-fruit is condemned.
The harvest season is the moment of reckoning — phthinoporinos captures that moment of exposure. All through spring and summer, the false teacher has appeared as a tree. But when fruit is required — when it matters — there is nothing. 'Twice dead' (Jude) suggests: dead in the root, dead in the fruit. True faith is always fruitful at harvest; phthinoporinos faith is seasonal pretense.