Yours, Etc.: Origins and Uses of 8 Sign-Offs
c.1400 as a noun, "person who sets malicious fires;" mid-15c. as an adjective, "capable of being used to set fires," from Latin incendiarius "causing a fire," from incendium "conflagration," from incendere "set on fire," figuratively, "incite, rouse, enrage," from in- "into, in, on, upon" (see in- (2)) + *candere "to set alight, cause to glow," related to candere "to shine" (see candle). Figurative sense of "enflaming passions" (adj.) is from 1610s. Military use, of bombs, shells, etc., attested from 1871. The obsolete verb incend is attested from c.1500.