Irregularity
Irregular, Disorderly
noun, な adjective
You know that 規則 means "rule" or "regulation." So a not "rule" or "regulation" is an irregularity.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
不規則な生活で体調を崩してしまった。
My health was ruined by my disorderly life.