Once more unto the brink

Once more unto the brink...

So, what's the origin of it? I say it all the time and know what it means, but I have no clue from where it comes.

That's a paraphrase of Shakespeare. See:

Perhaps it's not so much a paraphrase as an eggcorn - that is, a rationalisation of an unfamiliar word into something more familiar? Unless you are familiar with Shakespeare and/or pre-20th-century warfare, you are unlikely to know what a breach is, nor why anyone should urge people unto one. (VSD)