Where does "incensed" originate from?

Incense that you burn come from the Latin incendere, "to burn, to ignite." Likewise to be incensed, or to be enraged, derives from the same word though it is a past tense.

It shouldn't be difficult to see at least the superficial relation between the two. The emotions (and occasionally color) characteristic of someone that is enraged are similar to those of something being ignited: red, explosive, chaotic, hot.

At the root of each is excitation. Excitation of molecules => excitation of a person.

Though the adjective incensed meaning furious and incense, a substance used in churches because of its smell when burnt seem apart in meaning they have the same Latin source cendere and incendere meaning setting fire to something. "incensed" for furious is used figuratively. If someone is furious it is as if he is on fire. An image of comparison.

Incense is a substance that must be burnt to spread its particular smell. Latin incendere has the past participle incensum (neuter form). And this form is the basis for the aromatic substance incense.