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.