This term was introduced with the inception of the Eddie James All-Negro All-Stars in 1937. Originally used to refer to the chronic effects of a disasterous flim-flam, it later became used to describe Eddie's wife, Heather, who had slept with most of the band.
It also later got verbed and is used to describe saggy white moms with big asses getting reamed from behind.
"Once you go milf, you can't go back."
"This video shows your mom getting milfed."