Why do the English call condoms french letters?

Answer 1

French letter is a slang term, first used in the late nineteenth century, for condom. May be the general idea of the envelope keeping back fluids was at the origin of the idea. The French nickname for a condom is 'capote anglaise' ("English overcoat")

Answer 2

The origin of the term is not clear, so I can give only the most common explanations. The term 'french letter' by the way is now considered old-fashioned by the Brits themselves.

First of all there is the British tradition of ascribing things sexual (certainly the more 'disgusting' ones) to the French: syphilis was 'the French disease' and the kiss with exchange of bodily fluids 'French kissing'. So many Brits argue that a thing looking like a small square envelope containing a condom could easily have become a 'French letter' in slang usage.

Other explanations mention a Colonel Condum in the 17th century British army who first had preservatives distributed during service of his regiment in France to protect his men against French harlots. Some have argued that the word 'letter' has something to do with an antiquated meaning of the verb 'to let', namely 'to hinder, to prevent (veneral disease)'. The prefix French meaning of course again 'disgusting French habit'. Others again point at the habit of young Englishmen taking the Grand Tour of Europe in the 18th and 19th century, enclosing interesting little tidbits in ther letters home, which if sent from France would be condoms. Presumably these would not be the letters sent to their dear old mothers.

So take your pick. The French by the way used to call a condom an "English overcoat".