An agreement generally between two males or occasionaly two parties (I.E countries) in which an informal agreement is made as apposed to something that is signed and ratified and usually has no legal basis, it is simply a man giving his word that he will do something which is based on the traditional value of honour amongst
gentlemen. That should one man break his end of the agreement his Honour and trustworthiness will be called into repute and his status as a gentleman is finished. More modern language is likely used such as "losing your man card" for breaches of the agreement but the principle stays the same.
Has been used many times in the past
particularly during war amongst generals (who were considered men of honour) to broker such things as temporary ceasefires to evacuate wounded men, or the famous Christmas truce in WW1.
A more modern and simpler example would be if two guys were watching a
sports game and they decided to bet each other on the outcome. The agreement is informal but there is a gentlemans agreement to
follow through and pay up should one lose. Failure to pay on losing would be a breach of the gentlemans agreement and the loser runs the risk of being known as a bad payer and people would be less likely to bet with him in the future thus tarnishing his
reputation and his word
The Christmas truce of 1914 was a prime example of a gentlemans
agreement. That each man would lay his weapons down for the sake of the unofficial ceasefire, with each man effectively upholding a gentlemans
agreement that they would not attack
