c.1500, "resplendent" (obsolete), from Latin flagrantem (nominative flagrans) "burning, blazing, glowing," figuratively "glowing with passion, eager, vehement," present participle of flagrare "to burn, blaze, glow" from Proto-Italic *flagro- "burning" (cognates: Oscan flagio-, an epithet of Iuppiter), corresponding to PIE *bhleg-ro-, from *bhleg- "to shine, flash, burn" (cognates: Greek phlegein "to burn, scorch," Latin fulgere "to shine"), from root *bhel- (1) "to shine, flash, burn" (see bleach, v.). Sense of "glaringly offensive, scandalous" (rarely used of persons) first recorded 1706, probably from common legalese phrase flagrante delicto "while the crime is being committed, red-handed," literally "with the crime still blazing." Related: Flagrantly.