Flying monkeys is a reference to the flying monkeys in the Wizard of Oz (just to get the visual effect). If refers to a manipulation used by people with cluster B personality disorders (narcissism, borderline, antisocial, histrionic). If you cut off contact with them or do not do what they want, they will mobilize everyone around you to guilt, shame, bully, pressure you into doing their bidding.

Example:

You decide to forego the drama of the annual family Christmas Dinner. Your narcissistic mother begs, pleads, guilts you, shames you, threatens you - all to no avail. She then posts on Facebook, where you both have a lot of family and friends, “My son is turning his back on his family on the most important day of the year. My heart is broken! It’s the ONLY time I get to see him. What have I done so wrong?!” Everyone now turns to you and wants to know how you could do this to your poor mother??? They are coming to you with her message. You can hear her words coming out of their mouths. Flying Monkeys.