Erasable simply means that which can be removed from something as with an eraser: Things written on a blackboard are erasable.
Irascible, on the other hands, means easily angered; quick tempered: “My father has an irascible temper.”
The irascible temper of the professor often confused the students. Sometimes he would write information on the chalkboard which he would then consider erasable before the end of his lecture.
