Definition of "expletive"

No one entered more fully than Shakespeare into the character of this species of poetry, which admits of no expletive imagery, no merely ornamental line.

1839, Henry Hallam, Introduction to the Literature of Europe, volume 3, London: John Murray, page 501

deprecating being taken for ſerious, or to be underſtood that he meaneth any thing by them; but only that he uſeth them as expletive phraſes ... to plump his ſpeech, and fill up ſentences.

1683, Isaac Barrow, The Works of the Learned Isaac Barrow, London: M. Flesher for B. Aylmer, Against vain and raſh Swearing