Yours, Etc.: Origins and Uses of 8 Sign-Offs
1640s, "weak point of a sword blade" (contrasted to forte), from French foible (n.), from obsolete foible (adj.) "weak," from Old French foible "feeble," dissimilated from Lain flebilis (see feeble). Extended sense of "weak point of character" is first recorded 1670s. Related: Foibles.