Opinion

Latin opinio — belief, conjecture, reputation. From opinari — to think, believe. Always carried a sense of uncertainty — an opinion was a belief held without certainty, distinct from knowledge. The distinction has largely collapsed in common use.

'That's just your opinion' is used to dismiss established findings — a move that exploits the original sense (uncertain belief) to undermine what is actually well-evidenced. The phrase 'in my opinion' disclaims responsibility for a claim rather than honestly flagging uncertainty.