carte blanche (n.)

1707, "paper duly authenticated by a signature and otherwise blank, left to someone to be filled in at his discretion," French, literally "white paper," from carte (see card (n.1)) + blanche, from Old French blanc "white," a word of Germanic origin (see blank (adj.)). Figurative sense of "full discretionary power, unrestricted permission or authority in some manner" is from 1766. Compare blank check, used in the same figurative sense.