Irregular means not even, or not following the usual rule. An irregular shape, irregular hours, an irregular verb.
In grammar, an irregular verb does not form its past with -ed — go, went, gone — and English has about two hundred of them, which are the ones learners spend years on. Of a heartbeat, irregular is a clinical finding. Of a payment or an arrangement, irregular means not properly done and is a polite accusation. Note irregularity; and that irregular troops are fighters outside a regular army.
1adjectiveregularNot even or regular in shape or timing.
“The patient's heartbeat was irregular, so the doctor ordered more tests.”
2adjectiveirregularNot following the usual rule.
“English has about two hundred irregular verbs.”
irregular is a quality of unevenness.
Latin irregularis is in-, not, plus regularis, from regula, a rule — which was a straight stick used for ruling lines. Something irregular does not follow the straight edge.
Latin
irrēgulāris
Old French
irreguler
Middle English
irreguler
English, today
irregular
From Middle English irreguler, from Old French irreguler, from Medieval Latin - or Latin irrēgulāris, from in- + regularis, equivalent to ir- + regular.
Etymology from Wiktionary, CC BY-SA