2 Answers

Someone with a "hidden agenda" doesn't necessarily have an "axe to grind".  An agenda is a plan for a meeting....the issues that will be discussed and resolved.  If someone has a hidden agenda, that means he has his own plans about what is going to be discussed and resolved at that meeting.  An "axe to grind" means that someone has complaints to make against someone or something.  Grinding an axe sharpened it.  If someone has an axe to grind, I'd be careful of that person..He has something bothering him and he wants to complain about it.

To put the two together would mean (to me) that someone who is disgruntled about something(s) is planning to disrupt a meeting by directing discussions to the issues that bothers him.

ANSWER:

When Benjamin Franklin was a boy sharpening tools in his father's yard when a stranger carrying an ax came by and praised the boy on how good he was with the grindstone.  He then asked Franklin if he would show him how it would work on his own axe.  Once his axe was sharpened, the stranger simply laughed and walked away, giving young Franklin a valuable lesson about people with an "axe to grnd."