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.