early 14c., slabbe, "large, flat mass," thick enough not to be pliable, a word of unknown origin, possibly related to Old French escopel, escalpe "thin fragment of wood," which according to Klein is possibly a Gaulish word (compare Breton scolp, Welsh ysgolp "splinter, chip"). But OED rejects this on formal grounds.
In early use in English of iron, later of wood, stone, etc. The sense of "outer cut of a tree or log sawn up into planks or boards" is by 1570s. The 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).