c. 1400, "swelling, bulging," from Latin extensionem/extentionem (nominative extensio/extentio) "a stretching out, extension," noun of action from past-participle stem of extendere (see extend). In a concrete sense, "extended portion of something" (a railroad, etc.), from 1852. Telephone sense is from 1906.
interjection of pleasure, exultation, etc., by 1902; perhaps an extension and modification of hip (interj.).
1959, from clone (n.). Extension to genetic duplication of animals and human beings is from 1970. Related: Cloned; cloning.