Slang among nurses for sexual activity with a man having a reflexogenic erection, hence flexy. A reflexogenic erection is a form of unwanted erection of the male penis, that occurs when blood pressure changes in the abdomen, rather than by sexual arousal; It can be induced in a male by applying pressure to the lower abdomen, as in cases of forced sex/rape by females upon a male.
Reflexogenic erections are common among males who are paralysed and/or have spinal injuries.
Nurse1:" Sarah is pulling night duty on the Para Ward again?"
Nurse2:" Think she and that cute T8 are having some flexy fun?"