Webster's Revised Unabridged Dictionary · 1913

Irregular

Irregular , adjective

[Prefix ir- not + regular: compare French irrégulier.]

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. Compare Regular.
Mazes intricate, Eccentric, intervolved, yet regular Then most when most irregular they seem. — Milton
Leading the men of Herefordshire to fight Against the irregular and wild Glendower. — Shakespeare
A flowery meadow through which a clear stream murmured in many irregular meanders. — Jones

Irregular , noun

One who is not regular; especially, a soldier not in regular service.