Yours, Etc.: Origins and Uses of 8 Sign-Offs
1620s, Tuscan dialectal ditto "(in) the said (month or year)," literary Italian detto, past participle of dire "to say," from Latin dicere (see diction).
Originally used in Italian to avoid repetition of month names in a series of dates; generalized meaning of "same as above" first recorded in English 1670s. Dittohead, self-description of followers of U.S. radio personality Rush Limbaugh, attested by 1995. dittoship is from 1869.