1.
who; which one; which guy
A rough, rude way to say 'who' or 'which one,' referring to a person. Used when angry, frustrated, or speaking very casually among close friends. Should not be used in polite company.
Register
Very rough and rude. Part of the こそあど series at the vulgar level: こいつ (this one), そいつ (that one), あいつ (that one over there), どいつ (which one). These all refer to people in a dismissive or derogatory way.
Common expressions
どいつもこいつも is a fixed expression meaning "every last one of them" with a strong tone of exasperation or contempt.
POLITENESS SCALE (who/which person):