三七二十一

Almost never used alone - almost always inside the chunk 不管三七二十一 (bùguǎn sānqī'èrshíyī, 'regardless of the facts/consequences') or 管他三七二十一 ('who cares about anything'). The literal meaning is the multiplication-table fact; the figurative meaning is 'whatever the rules or consequences may be'.

Why 'three sevens'? One folk explanation: in Warring States era 苏秦 (Su Qin) calculated that '21,000 troops × 7 states' was the strength needed to oppose Qin - when the math 'didn't add up' it was thrown around as a careless dismissal. The phrase entered everyday speech as shorthand for 'never mind the calculations'.