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.