Not caring whether what you do is right or wrong. A principle here is a rule you hold yourself to, so a principled person refuses to do certain things even when they would gain by it — and an unprincipled person has no such line. It describes people (an unprincipled politician, an unprincipled businessman) and the things people do (unprincipled behaviour, an unprincipled attack on a rival). This is a serious accusation, and it is worth seeing exactly what it accuses someone of. It does not say they are cruel or that they enjoy hurting people; it says they are willing — that nothing inside them stops them if lying or cheating happens to be the quickest way to what they want. That is why it so often attaches to people with power, and why it sits so naturally in politics and business writing. The word is formal and rather literary; in ordinary speech people say "he has no morals" or "he'd do anything". Its opposite, principled, is a real and common word of high praise, and the pair is worth learning together.

毫無原則的、不擇手段的、沒有道德底線的。台灣講「不擇手段」語感最接近——重點在「為了目的什麼都做得出來」，而不是「壞心」。