不規則
ふきそく · fukisoku · JLPT N2 · na-adj
irregularity
不規則 = 不 (not) + 規則 (rule): not following the rule, so unsteady and irregular
不 means not and 規則 means rule or regular pattern. Put together, 不規則 names anything that does not keep to a regular pattern, such as an irregular heartbeat or an irregular verb.
不 — single pictograph; bird flying upward
Originally a pictograph in oracle bone script, traditionally interpreted as a bird flying up toward the sky and unable to come down, suggesting negation or refusal. Over time it became the standard negation word in Chinese, used before verbs and adjectives to mean 'not'.