slat (n.)

late 14c., earlier sclat (c. 1300), "a roofing slate, a thin, flat stone," from Old French esclat "split piece, chip, splinter" (Modern French éclat), back-formation from esclater "to break, splinter, burst," probably from Frankish *slaitan "to tear, slit" or some other Germanic source (compare Old High German slizan, Old English slitan; see slit (v.)). Meaning "long, thin, narrow piece of wood or metal" attested from 1764.