The phrase "cozy flannel" refers to a soft and warm fabric made of brushed cotton, often used in clothing or bedding. It means something comfortable and snuggly that makes you feel warm and relaxed. Full definition
The shirt has a flair and elegance that you will not find in any other warm and cozy flannel shirt. (garnethill.com)
But the plaid I'm making a case for isn't the typical cozy flannel you might have in mind — rather it's the revival of the classic 70s plaid. (atelierdore.com)
I linked to some more cozy flannel options below as it makes the perfect pairing. (maggiealamode.com)