This was a new addition to his expletory vocabulary, which had accrued from Ned Burnleigh's companionship.
1610s, originally "a word or phrase serving to fill out a sentence or metrical line," from Middle French explétif (15c.) and directly from Late Latin expletivus "serving to fill out," from explet-, past participle stem of Latin explere "fill out," from ex- "out" (see ex-) + plere "to fill" (see pleio-).
Sense of "exclamation," often in the form of a cuss word, first recorded 1815 in Sir Walter Scott, popularized by edited transcripts of Watergate tapes (mid-1970s), in which expletive deleted replaced President Nixon's salty expressions. As an adjective, from 1660s.
mid-15c., from Latin expletivus (see expletive (n.)).
Any exclamation or oath, especially one that is obscene or profane, as in “Dammit, I forgot to buy the milk.”
Note: The Oval Office tapes of President Richard Nixon, released during the investigation of the Watergate scandal, made famous the phrase “expletive deleted,” which appeared frequently in expurgated transcripts of the tapes.