1610s, "a word or phrase serving to fill out a sentence or metrical line," from French explétif (15c.) and directly from Late Latin expletivus "serving to fill out," from explet- past-participle stem of Latin explere "fill out, fill up, glut," from ex- "out" (see ex-) + plere "to fill" (from PIE root *pele- (1) "to fill").
Sense of "an exclamation," especially "a curse word, an oath," 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., in grammar, "correlative," from Latin expletivus "serving to fill out" (see expletive (n.)).