Not regular; not conforming to a law, method, or usage recognized as the general rule; not according to common form; not conformable to nature, to the rules of moral rectitude, or to established principles; not normal; unnatural; immethodical; unsymmetrical; erratic; no straight; not uniform; as, an irregular line; an irregular figure; an irregular verse; an irregular physician; an irregular proceeding; irregular motion; irregular conduct, etc. Cf. Regular.
One who is not regular; especially, a soldier not in regular service.
I spent 20 years doing research on regular and irregular verbs, not because I'm an obsessive language lover but because it seemed to me that they tapped into a fundamental distinction in language processing, indeed in cognitive processing, between memory lookup and rule-driven computation. Steven Pinker