slab (n.) Look up slab at Dictionary.com
late 13c., "large, flat mass," of unknown origin, possibly related to Old French escalpe "thin fragment of wood," which seems to be a Gaulish loan word (cf. Breton scolp, Welsh ysgolp "splinter, chip"). But OED rejects this on formal grounds. Meaning "rectangular block of pre-cast concrete used in building" is from 1927.