You must live with a person to know a person.
Note: Literal translation — ‘No knowledge until cohabitation.’ Ancient Roman citizens learned this wisdom from conscription into the Roman legions. “Homini ne fidas, nisi cum quo modium salis absumperis.” (Do not trust a man unless it is one with whom you have consumed a measure of salt.) The measure of salt is a reference to the preserved meat a soldier would consume in the field. Over a long campaign, one learned whom one could trust in the intimacy of close quarters. Living together over long periods of time naturally reveals one’s true character. Ask anyone who has been married for a while.
Note also: You might expect this proverb to be written, ‘Ní haithne go haontíos.’ This is because the Irish idiom for ‘I don’t know someone’ is ‘Ní aithne agam ar duine éigin.’ But the word ‘aithne’ literally means ‘acquaintaince.’ So ‘knowing’ in this sense is possession of superficial information about a person, like a person’s name and occupation. A deeper ‘knowing’ is conveyed by the word ‘eolas.’ Tá aithne agus eolas agam air. (I know and understand him.) There is a third type of ‘knowing’ in Irish. ‘Tá a fhios agam.’ Ach sin scéal eile.