The word expletive comes from the Latin verb explere, meaning "to fill", via expletivus, "filling out". It was introduced into English in the seventeenth century to refer to various kinds of padding — the padding out of a book with peripheral material, the addition of syllables to a line of poetry for metrical purposes, and so forth. Use of expletive for such a meaning is now rare. Rather, expletive is a term in linguistics for a meaningless word filling a syntactic vacancy (syntactic expletives). Outside linguistics, the word is much more commonly used to refer to "bad language". Some linguists use it to refer to meaningless, "filler" use of "bad language" ("expletive attributives"), distinguishing this from meaningful use.
Syntactic expletives are words that perform a syntactic role but contribute nothing to meaning. Expletive subjects are part of the grammar of many non-pro-drop languages such as English, whose clauses normally require overt provision of subject even when the subject can be pragmatically inferred (for an alternative theory considering expletives like there as a dummy predicates rather than a dummy subject based on the analysis of the copula see Moro 1997 in the list of references cited here). Consider this example:
Following the eighteenth-century conception of pronoun, Bishop Robert Lowth objected that since it is a pronoun, it should have an antecedent. Since it cannot function like that in Latin, Lowth said that the usage was incorrect in English.
Whether or not it is a pronoun here (and linguists today would say that it is one), English is not Latin; and the sentence was and is fully acceptable to native speakers of English and thus was and is grammatical. It has no meaning here; it merely serves as a dummy subject. (It is sometimes called preparatory it or prep it, or a dummy pronoun.)
Bishop Lowth did not condemn sentences that use there as an expletive, even though it is one in many sentences, for example:
The nomenclature used for the constituents of sentences such as this is still a matter of some dispute, but there might be called subject, are copula, and ten desks predicate nominal. Meanwhile here is an adverbial phrase that conveniently reveals the semantic vacuity of there in this example.
There is some disagreement over whether the it in such sentences as
are expletives. Whereas it makes no sense to ask what the it means in "It is important that you work hard for the exam", some people might say that the dummy it in "It is raining now" means the weather (even if the word weather has not previously been mentioned). Thus the it in such sentences is sometimes called expletive, sometimes a weather "it". Compare with weather verb.
In sentences such as
bloody contributes nothing to the meaning. Rather, it suggests the strength of feeling (usually anger or irritation, but often admiration, etc.) of the speaker. In having no meaning, it resembles the syntactic expletives discussed above; in these uses, bloody is an expletive.
Expletive attributives common in English include damned, fucking, and motherfucking. Note that not all uses of such words are of this kind: "The concert was damned good" contains an expletive attributive, but "He was damned by the photographic evidence" does not as the possibly offensive word is used meaningfully. (However, see below on "bad language".)
Other words that are never thought of as offensive can be used in similar ways. For example:
The phone line discussed may (before it was cut off) have been just as good as any other, and therefore would not have been wretched in the dictionary senses of "extremely shoddy", "devoid of hope" or similar. Rather, wretched serves here as a politer equivalent of expletive bloody and the like. However, such meaningless uses of inoffensive words are seldom referred to as "expletive".
This sense became popular when transcripts of Richard Nixon's internal tapes were made public. The phrase "expletive deleted" was put into the court record when the notoriously profanity-laced discussions with H. R. "Bob" Haldeman and other Watergate insiders went beyond the bounds of common decency. The phrase entered the public imagination to the point where protestors outside the White House held up picket signs reading, "IMPEACH THE CUNTS!" As the tapes were declassified over the years, and clips of them were aired on television for documentaries, the word "goddamned" appeared to account for a majority of the references to "Expletive Deleted."
In later years, the phrase expletive deleted became commonplace as an ironic expression that indicates that a profanity has been omitted and passed into general usage as a convenient linguistic figleaf.