What does get on her mean relating to girlfriends?
When a dude says get on her, relating to a girl he means that he's basically trying to get to know her. He's interested in her and he wants to start talking to her as more than a friend. Stupid answer
A wag is a mischievous person, a jokester. WAG is a very crude acronym for 'wise ass guess'. More commonly, it is used in the British press to mean "Wives and Girlfriends", specifically relating to futbol players' WAGs. It has now spread to most other sports. Or it could mean to skip lessons at school.