irregular adj.
1 not regular in size/shape/frequency
VERBS be, look | become
ADV. highly, very | rather, slightly, somewhat The cells are slightly irregular in shape.
2 not allowed according to the rules
VERBS be, seem, sound
ADV. highly, most, very It would be highly irregular for a police officer to accept money in this way. | slightly, somewhat some somewhat irregular business practices
You can also check other dicts: irregular (English, 中文解释 ), wordnet sense, Collins Definition