but that Definition

but¹
conj

1. Contrary to expectation.

Example: She fell down but didn't hurt herself
Thesaurus: however, nevertheless, still, yet, though, although.
2. In contrast.

Example: You've been to Spain but I haven't
3. Other than.

Example: You can't do anything but wait
4. Used to emphasize the word that follows it.

Example: Nobody, but nobody, must go in there

prep

1. Except.

Example: They are all here but him
Thesaurus: save, aside from, except, bar.

adverb

1. Only.

Example: I can but try
Thesaurus: merely, simply, barely, solely, purely, just, no more, exactly, no other than, without, only.

noun

1. An objection or doubt.

Example: no buts about it
Example: too many ifs and buts

Idiom: but for

Were it not for; without.
Example: I couldn't have managed but for your help

Idiom: but that

rather formal or old use
Were it not that; except that.
Example: There seemed no explanation but that he had done it

Etymology: Anglo-Saxon butan, meaning ‘outside of' or ‘without', from be by + utan out.

Info:

Although it is sometimes regarded as poor style, it is not ungrammatical to begin a sentence with but. Indeed, many writers have done so with considerable effect. It is also common in conversation and in forms of English in which a looser grammatical structure is appropriate.