mid-14c., "temporary wooden framework upon which workmen stand in erecting a building, etc.," a shortening of an Old North French variant of Old French eschafaut "scaffold" (Modern French échafaud), probably altered (by influence of eschace "a prop, support") from chaffaut, from Vulgar Latin *catafalicum.
This is from Greek kata- "down" (see cata-), used in Medieval Latin with a sense of "beside, alongside" + fala "scaffolding, wooden siege tower," a word said to be of Etruscan origin.
From late 14c. as "raised platform on a stage in a play;" the general sense of "viewing stand" is from c. 1400. The meaning "platform for a hanging" is from 1550s (as a platform for a beheading from mid-15c.). Dutch schavot, German Schafott, Danish skafot are from French.
As a verb from mid-15c., scaffolden, "construct a scaffold;" by 1660s as "put a scaffold up to" (a building).