Afirmative sexual availability.
If one is single, not in a monogamous relationship, or otherwise not exclusively attached, they are said to be fairdinkum. (it's fair to dink 'um)
To pork someone who is not 'available' would be unfairdinkum.
John has the hots for his best friend's girlfriend Sheila, but to bone her behind his buddy's back would be unfairdinkum. John must tell his friend that he saw Shiela hoing it up with the bouncer at a singles club, and later chuggin' cock in the back alley for free shooters. His friend should then break up with the ho. Now she's fairdinkum, and John can tap dat ass without fear of reprisal.
Linda: "Tom is so hot, I wanna ride him like a Jersey steer. Too bad he's dating Cindy... she'd rip my uterus out with her bare hands if she found out I boned him."
Keri: "You're in luck! As of last night, Tom and Cindy are on a 'break' ... he's fairdinkum!"
An Australian expression of disbelief.
John: "I got really pissed last night"