The same as shaft worthy or fuckable. A shag worthy girl or boy is attractive (pretty, sexy etcetera) enough for you to want to have sexual intercourse with her/him, but most oftenly you wouldn't seriously consider initiating a relationship with her/him. The number of shag worthy babes/hunks usually increase proportionally with your consumption of alcoholic liquid. To be shag worthy doesn't imply you have to be good-looking or sexy, not at all, it simply implies you are capable to have sex with the person in an especially needy situation.
- Mate, how's the party last night? Got laid?
- Euhm, to be honest I didn't have that much of a success with the babes yesterday. But you know, with all that vodka...I finally just grabbed a chick I found shag worthy. It wasn't that bad actually, she even gave me oral sex!