May derive from a Latin Io, with the I written over the o, placed at the end of an exclamation to mark it as such. The Io may either be an abbreviation for interiectiō(“interjection”), or else the interjection iō(“hey!”). It expresses admiration in the sentence clause(s), in the Latin sense, of awe or wonderment.
2008, June, A Michele Funston, "Taxonomic Revision of Roldana (Asteraceae: Senecioneae), a Genus of the Southwestern U.S.A., Mexico, and Central America", Annals of the Missouri Botanical Garden, volume 95, issue 2, page 307:
TYPE: Mexico. Nayarit: Tepic, 10 Feb. 1927, Marcus E. Jones 23357 (holotype, POM not seen; isotype, US!).
(Internet, fandom slang)Used to join a descriptor, such as a trait, with its referent, usually a character or relationship.
She particularly prefers to read fanfiction about hurt!Spock.
2000 октобар 3, Alli Snow, “comment and question”, in alt.tv.stargate-sg1, Usenet, message-ID <firstname.lastname@example.org>:
But it has been a while since we've seen Smart!Jack; I wouldn't mind seeing him again...
2000 децембар 12, Laura, “Since You've Been Gone”, in alt.tv.x-files, Usenet, message-ID <3A36ABBA.FE77CD07@erols.com>:
Some take great exception to Action!Scully thinking she should be a little more country than rock and roll in her delicate condition.