late 14c., "mark off verse in metric feet," from Late Latin scandere "to scan verse," originally, in classical Latin, "to climb, rise, mount" (the connecting notion is of the rising and falling rhythm of poetry), from PIE *skand- "to spring, leap, climb" (source also of Sanskrit skandati "hastens, leaps, jumps;" Greek skandalon "stumbling block;" Middle Irish sescaind "he sprang, jumped," sceinm "a bound, jump").
Missing -d in English is probably from confusion with suffix -ed (see lawn (n.1)). Sense of "look at closely, examine minutely (as one does when counting metrical feet in poetry)" first recorded 1540s. The (opposite) sense of "look over quickly, skim" is first attested 1926. Related: Scanned; scanning.