Scaffolding is the temporary metal frame workers climb on outside a building. Poles, boards, and a ladder up the middle.
It goes up before the work and comes down after it, which is the whole idea, and that is why teaching borrowed the word: scaffolding a lesson means giving learners temporary support that is taken away as they get stronger. It is a mass noun, so you put up scaffolding rather than scaffoldings. A single upright structure is a scaffold, though that word carries a much grimmer sense as the platform for a hanging.
1nounscaffoldingA temporary framework of poles and boards used by builders.
“Scaffolding covered the whole front of the house.”
2nounsupportTemporary support given to a learner and gradually removed.
“Good teaching provides scaffolding and then withdraws it.”
Middle English
scaffaldyng
English, today
scaffolding
Middle English scaffaldyng, skaffaldyng, equivalent to scaffold + -ing.
Etymology from Wiktionary, CC BY-SA