"Once and For All'

I know it's an expression. I know it probably found its way into common venacular hundreds of years ago, when the english language was different, but why do we (including myself) say "once and for all"?

As it is said and written we all would appear, at least to me, to use it incorrectly. We use it to mean, more-or-less, 'finally'. For example: 'I am going to find out if she likes me once and for all'. But 'all' doesn't imply longevity, it implies quantity. Therefore this statement would seem to break down to "one time and for everyone".

Is the expression short for 'once and for alWAYS'? can you really abbreviate a sentence? Is that allowed?

Anyway, it just seems to me that 'once and for all' is something the Three Musketeers would say.

On the same note, why do people say 'for good' in context of 'forever'? For example: 'I am quitting college for good.' How did this expression come to be? Couldn't quitting college be considered 'bad'?