I'm watching the TV drama 'sex education'. In S01E01, a guy says to his friend 'Everybody is either thinking about shagging, about to shag, or actually shagging, and you can't even jack your beanstalk'. What does 'jack your beanstalk' mean?
It is not a standard expression, but given the context is probably a joke euphemism for "masturbation". "Jack off" is a slang for masturbation. The expression plays on the title of the children's story "Jack and the Beanstalk"
It's a funny way of saying masturbate, without sounding too dirty or serious.