source: Hamazura Shiage | Wiki | Anime Amino

1. not even or balanced in shape or arrangement:

〈形などが〉ふぞろいの, 〈表面などが〉でこぼこの, 平らでない(uneven); まっすぐでない

e.g. his strong, irregular features.

occurring at uneven or varying rates or intervals:

〈時間･出来事などが〉不規則な, 不定期の; 変則的な, 異常な

e.g. an irregular heartbeat.

Botany (of a flower) having the petals differing in size and shape; zygomorphic.

2. contrary to the rules or to that which is normal or established:

〈行動･生活などが〉乱れた; 反則の, 法［道徳］に反する

e.g. they were questioned about their involvement in irregular financial dealings.

(of troops) not belonging to regular or established army units.

〘軍〙 〈兵などが〉正規でない

3. Grammar (of a verb or other word) having inflections that do not conform to the usual rules.

〘文法〙 不規則変化の

noun (usually irregulars)

1. a member of an irregular military force.

〖通例～s〗 不正規兵［軍］

2. mainly North American an imperfect piece of merchandise sold at a reduced price.

late Middle English (in the sense ‘not conforming to rule (especially that of the Church’)): via Old French from medieval Latin irregularis, from in- ‘not’ + regularis (see regular).