used to join coordinate sentence elements of the same class or function expressing contrast: on the other hand
meant to tell you but forgot
and nevertheless; yet
were poor but proud
used to introduce an expression of protest or enthusiasm
But that's ridiculous!
used to signal that a new topic is being embarked on
But to continue …
formal were it not; save
He would have collapsed but for your help
formal (in negative contexts) without the necessary accompaniment that
They never meet but there's a skirmish of wit between them — Shakespeare
dialect otherwise than; that … not
That was how I was brought up, and I don't know but what it was just as good a way as all these new-fangled notions for training children — L M Montgomery
[Old English būtan (prep and conj) outside, without, except, except that, from Old English be by1 and ūtan outside]