1570s, "pertaining to a plague," from plague (n.) + -y (2). Figurative meaning "vexatious, troublesome" is from 1610s. As an adverb, "vexatiously, deucedly" (properly plaguily) it is attested from 1580s, often with deliberate attempt at humor. Johnson also has woundy "excessive." The sense of "plague-stricken, marked by the plague" (c. 1600) is now archaic or obsolete.