Mange - a skin disease of mammals caused by parasitic mites and occasionally communicable to humans. It typically causes severe itching, hair loss, and the formation of scabs and lesions.
Knob - penis, dick, cock
When some stupid asshole of a guy irritates or pisses you off he is a mangey-knob
feminine = mangey-fanny
Me: that idiot just cut me off
Friend: what a mangey-knob