Definition - The part of speech that contains the set of all exclamations.

Example -

(1) Darn! (interjection of annoyance)
(2) Ugh! (interjection of disgust)
(3) Darnugh! (interjection of disgusted annoyance) [rare]

Etymology -
The word derives via Middle French from the Latin interjectionem, a throwing or placing between (from inter, between + jacere, to throw).
Note: Interjections were so named because they are interjected between sentences, clauses, or other words that usually have no other grammatical connection to them.

Oxford English Dictionary -
Its first citation in this sense is from about 1530:
"Interjection be suche as serve to expresse the passyons and the affections of the mynde."
(Jehan Palsgrave, Lesclarcissement de la langue françoyse, 149 )

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