The origin of this phrase (Which means excuse my use of profanity) does not
originate for the French word for seal, as Lazy
Time Waster suggests.
It in fact originates from the constant warfare between England and France many years ago, at that time "French" was
associated with indecent things and activies (Swearing, kissing etc).
This was done fairly often in those days, sexually transmitted diseases (for one thing) had different names in different
countries.
What the English might call the
French disease, the Dutch might call the Spanish disease,
the French might call the English disease and so on.