Èhuài le de háizi bùguǎnsānqī'èrshíyī, zhuāqǐ mántou jiù chī.
The starving child grabbed the bun and ate without a second thought.
Tips
history
Recorded in Feng Menglong's Ming-dynasty 警世通言 (Jingshi Tongyan, Stories to Caution the World). Why 'three sevens are twenty-one'? Because 3x7=21 is the most basic, undeniable truth, so to 'not care that 3x7=21' means to ignore even the simplest reasoning. To act first, think later.
usage
Very common in spoken Chinese. Often pairs with 就 (jiu): 不管三七二十一 + 就 + verb. Has a slightly negative or impulsive flavor; not used to praise careful action.
