expletiveyoudaoicibaDictYouDict[expletive 词源字典]
expletive: [17] Originally, an expletive word was simply one used to ‘fill up’ a line of verse, to complete its metrical pattern (expletive comes from Latin explētus, the past participle of explēre ‘fill out’, a compound formed from the prefix ex- ‘out’ and plēre ‘fill’, source of English complete and related to English fill).

Hence the term came to be used for a redundant word, not contributing anything to the meaning of the sentence: “The Key my loose, powerless fingers forsook”, a lame and expletive way of saying “I dropt the key”, Robert Southey 1804. The first recorded example of its euphemistic application as a noun to ‘profanities’ is by Sir Walter Scott in Guy Mannering 1815: ‘retaining only such of their expletives as are least offensive’.
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expletive (n.)youdaoicibaDictYouDict
1610s, "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, fill up, glut," from ex- "out" (see ex-) + plere "to fill" (see pleio-).

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.
expletive (adj.)youdaoicibaDictYouDict
mid-15c., in grammar, "correlative," from Latin expletivus "serving to fill out" (see expletive (n.)).