Yours, Etc.: Origins and Uses of 8 Sign-Offs
early 15c. (implied in externalle), from Middle French externe or directly from Latin externus "outside, outward" (from exterus; see exterior) + -al (1). This version won out over exterial. Related: Externally.
external ex·ter·nal (ĭk-stûr'nəl)adj. Abbr. ext. Relating to, connected with, or existing on the outside; exterior.