Excuse my French
Meaning to forgive one for using a swear word / cussing. This is a phrase used when one tries to pass swearing off as french language.

This is from a website visitor:
further notes: the expression comes from England, it does refer to the use of swear words. it was a way of demonising England's enemy of the time. also the French disease - syphilis England also went to war with the Dutch, which is where the phrases Dutch courage, and double Dutch come from ... Dutch courage - the only way the Dutch can get courage to fight is through drink. Double Dutch - talking nonsense.

(来源：英语麦当劳－英语杂志 http://www.EnglishCN.com)