What is the Libertarian definition of "arms"?

I'm going to leap to the assumption that you are referring to the 2nd Amendment. That being the case, the purpose of it is to guarantee that the people have the right to own weapons, so they can bear them in service of the militia if needed. That's why it references the RIGHT of the PEOPLE to own weapons, and also a well regulated militia.

So in this instance, Arms refers to weapons that would be suitable for use by a citizen militia. In today's world, that would be things like pistols, shotguns, AR-15s with high capacity magazines, etc. Weapons like those.

As a card carrying, dues paying Libertarian it means just what out Founding Fathers meant: Something to defend yourself with.

I find no definition of arms pertaining to the constitutional right to bear arms. Does this mean I should have the right to have a tactical nuclear weapon in my basement?

arm

plural noun: arms

1.

each of the two upper limbs of the human body from the shoulder to the hand.

Weaponry, guns

Appendages which hang from your shoulders.

two of those things similar to legs