This is based on imagery, where patience is like a rope with which you are holding on to an aggravating situation. Imagine that the situation (or the people involved) is pulling hard against your "rope" and that it is slowly snapping strand by strand and wearing thin until the last "strand" of the "rope" of your patience snaps with persistent aggravation, and you just blow up and go berserk.

It means that you are running out of patience and that someone or something is making you angry! It also meand that you've been patient so far but that that patience is coming to an end. Hope that helps!

ok