Used (usually) by heterosexual (mostly chauvenist) males when describing relatively easy sex or easy females. In other words, there is/was little, or no, effort (such as dinner, movie, time, money, favors, etc.) to receive sex whether regularly, or on a date, etc.
I love having Lana on the side. She's low maintenance and delivers the straight goods. No need for dinner, expenses and all that stuff, we just do it right away and that's it.