1840, "shape and size" (of a book), via French format (18c.), ultimately from Modern Latin liber formatus "a book formed" in a certain shape and size, from past participle of formare "to form," from forma "form, shape" (see form (n.)). Extended to computers by 1955.
"arrange into a format," 1964, in reference to electronic computing, from format (n.). Related: Formatted; formatting.