What's the difference between 'fellow' and 'prig' in connotational meaning? [closed]

What's the difference between 'fellow' and 'prig' in this sentence from Michael and Mary (1930) written by A. A. Milne?

Then tell me: Why are you the devil of a fellow if you like drinking whisky, and the devil of a prig if you don't?

"The Devil of a fellow" means an excellent fellow; a hell of a chap!

It's old-fashioned slang.

We vied with one another in caring for our platoons, and, off duty, carried ourselves with no end of swagger, each trying to be the devil of a fellow. - A Subaltern's War, Charles Edmonds, 1929

The devil of a fellow

The devil of a chap

Then tell me: Why are you thought of as a great bloke if you like drinking whisky, and a hell of a prig if you don't?

"Fellow " and "prig" are not really related. The relation is through the phrase "The Devil of a...".

"The Devil of a fellow" means "A bad man" (essentially "A devil like man" although it is usually applied to people much less bad than the devil). "The Devil of a prig" means a very bad prig, meaning someone more priggish than an ordinary prig. (A prig being someone who is overly strict in opposing any form of vice or pleasure.)

So a man who likes whiskey is thought to be a bad man, but someone who doesn't is an excessive killjoy.