A vague definition in a dictionary, "shag：a sexual partner of a specified ability". Is there any better or plainer explanation?

I'm not a native English-reader, I'm Chinese. So mostly I get meanings of words by consulting dictionaries. I read this in a dictionary about the word shag:

a sexual partner of a specified ability.

Sexual partner is quite understandable to me, but what does "of a specified ability" refer to? Sexual ability?

Could there be any better explanation which would give me a plainer idea of it or are there words synonymous with shag?

Solution 1:

Shag as a noun as defined in your question is not used without qualification. You could say:

She was a great shag, goes like the clappers.

Or

He was a crap shag, too much teeth.

But not

*They were a shag.

A word with similar usage patterns is lay. In fact the definition is the same.