FROM STRONG’S GREEK DICTIONARY (1890)

Strong’s Greek Dictionary (1890) — public domain, printed whole.

phtheiro φθείρω — fthi'-ro — probably strengthened from phthio (to pine or waste); properly, to shrivel or wither, i.e. to spoil (by any process) or (generally) to ruin (especially figuratively, by moral influences, to deprave):--corrupt (self), defile, destroy.