late 14c., scannen, "to mark off verse in metric feet, analyze verse according to its meter," 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").
English lost the classical -d- probably by confusion with suffix -ed (compare lawn (n.1)). Intransitive meaning "follow or agree with the rules of meter" is by 1857. The sense of "look at point by point, examine minutely (as one does when counting metrical feet in poetry)" is recorded by 1540s. New technology brought the meaning "systematically pass over with a scanner," especially to convert into a sequence of signals (1928). The (opposite) sense of "look over quickly, skim" is attested by 1926. Related: Scanned; scanning.
1706, "a close investigation, an act of scanning," from scan (v.). The meaning "image obtained by scanning" is from 1953.