Irascible
Etymology
From Latin irascibilis, "prone to anger," from the verb irasci, "to grow angry," which is built on ira, "anger, wrath" — the root behind "irate" and "ire." The Latin form specifically named a capacity for anger, a standing readiness to flare up. English kept that precise nuance: not angry at the moment, but easily made so.
adjective · /ih-RAS-uh-bul/
Easily angered; having a hot, quick temper.
What it names
The disposition of the person you handle carefully because you never know what small thing will set them off.
Example
The old professor was brilliant but irascible, and students learned never to arrive late to his lectures.
Related idea
Irate describes someone actually angry right now; irascible describes the underlying temperament that makes anger come easily.