Shag is a British slang term for sexual intercourse.
It is not too vulgar and can therefore be used in informal settings.
Amusingly it is also the name of dance step in the USA.
This has led to many disappointed youngsters who, hearing that their visitors favourite past-time is shagging, wrongly get their hopes up.
It is also the scientific name for a species of several cormorants, especially Phalacrocorax aristotelis of Europe and North Africa.
“Fancy a shag?”
“Do you prefer the shag to the foxtrot?”
“That cormorant is definitely a shag!”