Truthing: Telling the truth, especially an unpleasant or unwelcome truth, about someone or something.
Dan Savage wrote in The Stranger last week about the case of anti-gay crusader George Rekers, who traveled to London and Madrid for 10 days with a gay male escort, Jo-Vanni Roman; Savage compared the situation to the outing in 2006 of Ted Haggard, then the president of the National Association of Evangelicals, by male prostitute Mike Jones:
Jones was left feeling bitter and broke after he did gays and lesbians everywhere a huge service stepping forward and outing/truthing Haggard.
I couldn't find a definitive origin for the contemporary usage of truthing. (The parallel outing—in the sense of "exposing someone's undeclared homosexuality"—first appeared in print in 1990, according to the OED, although other senses of out as a transitive verb date back to the 16th century.) Urban Dictionary has a March 2005 citation for truthing ("to tell the truth about something OR slang for hardcore truth or dare"). There is an undeniable gap in the English language that invites truth to be used as a verb. "Unlike lie (v.), there is no primary verb in English or most other IE [Indo-European] languages for 'speak the truth'," says the Online Etymology Dictionary.
And it turns out that truthing predates the 21st century by a long stretch. The OED gives an 1881 citation in a publication called English Mechanic: "It permits of the removal of such Rails for any truthing which they may require." The OED's definition is "to bring to 'truth'" or "to adjust accurately, to true."
Ground truthing (or groundtruthing) is used in scientific contexts to mean "the process of sending technicians to gather data in the field that either complements or disputes airborne remote sensing data collected by aerial photography, satellite sidescan radar, or infrared images." (Source: Mission Ground Truth.)
Truthiness ("the quality of preferring concepts or facts one wishes to be true, rather than concepts or facts known to be true") was the American Dialect Society's 2005 Word of the Year. It was popularized by the satirist Stephen Colbert, but the ADS noted that other meanings of the word date back to 1824. The OED provides citations for other truth variations, including truthly, truthify, thruthsman, and truthhead.
UPDATE: Here's Mark Peters on truther, "a term for folks who reject the standard explanation for the events of 9/11 in favor of a thin web of speculation that includes the U.S. government colluding in the extravagant murder of thousands of its own citizens."
As the lady says, don't be lyin' when you oughta be truthin':
Video (1966) from Daily Motion. Lyrics by Lee Hazlewood.