A woman with an intense, gripping, white knuckle fear of rejection that has led her to spend all of her time with men who have no reason to reject her sexually. This behavior gives the woman a false sense of security for as long as she and her "fag" engage in thoroughly demolishing each other's chances at forming real romantic relationships.
Hag: Omigod, Tim called me at 9:15 last night instead of 9:00! What was he doing for those fifteen minutes?
Fag: Probably cheating on you, girlfriend! You don't need him. Text him a Dear John and we can go shopping!
Hag: What would I do without you? It pays to be a fag hag!
An insult aimed at J.K. Rowling by wannabe literature snobs who like to spend all their time critiquing teen fantasy books because they're bitter that nobody wants to publish their own rubbish. The root of the insult is that J.K. Rowling uses a lot of the plot device deus ex machina (in this case, going back and changing the ending as in reversing time to fix a problem or adding/changing a detail hence "just kidding!"), widely considered a literary cop-out but generally accepted in literature intended for children by all but the most persnickety critics.
Hermione goes back in time in Harry Potter book 3 so that Sirius Black can escape prison.