I am having trouble understanding the sentence below.
"She pretends to be your best friend, but all the while she's having it on with your boyfriend."
Does this mean she is betraying you by dating your boyfriend?
I suspect three idioms are being confused here:
1. having it off with (someone) - this means more than just dating - also sleeping together
2. carry/carrying on with (someone) - having a relationship with someone, with the implication they will also be sleeping together
3. having me/him on - to persuade someone that something is true when it is not, usually as a joke.
Of course, often the person will twig (=realize what's going on) and so say, "Are you having me on?"
Student or Learner