屎(shǐ) - excrement, faeces, mess in Mandarin

屎 (shǐ) is the crude everyday word for excrement. It is the register of playground jokes and dog walkers, and it is common enough in speech that learners meet it early, usually in 狗屎 (dog mess) or in the verb phrase 拉屎 (to defecate).

The polite alternatives matter more than the word itself. 大便 (dàbiàn) is the neutral term you use with a doctor, on a form, or with anyone you have just met. 上厕所 (going to the toilet) covers most of what an English speaker would say indirectly anyway. Choosing 屎 in those settings is not rude exactly, but it marks you out the way "shit" would in a British surgery.

The trap is the figurative use. 狗屎 works as a dismissal of something worthless, close to "rubbish", but 屎 on its own does not carry the huge range English gives to its equivalent. There is no Mandarin sentence built on 屎 that means bad luck, nonsense or a difficult situation. For those you want 倒霉, 胡说 and 麻烦 respectively.