Do you know ghouls from goblins and ghosts?
late 13c., "large, flat mass," of unknown origin, possibly related to Old French escopel, escalpe "thin fragment of wood," which according to Klein is possibly a Gaulish 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. Slab-sided is "having flat sides like slabs," hence "tall and lank" (1817, American English).