What is the X in X-mas?
1570s, from obsolete flecche "to bend, flinch," probably from Old French flenchir "to bend," probably from Frankish *hlankjan or some other Germanic source (cf. Middle High German linken, German lenken "to bend, turn, lead"), from PIE root *kleng- "to bend, turn" (see link (n.)). Related: Flinched; flinching. As a noun, from 1817.