But (conjunction)

1. Used to introduce a phrase or clause contrasting with what has already been mentioned.
2. Used to indicate the impossibility of anything other than what is being stated.
3. Used to introduce a response expressing an objection or difficulty.
4. Used to introduce a statement that reduces the force or importance of what precedes it.

Origin:

Old english būtan, from bē- (as a verbal prefix) + ūtan 'out', from ut 'out, outside', from proto-germanic *ūt.

Examples:

1. She wanted to go to the party, but she had too much work to do.
2. I have no choice but to accept the job offer.
3. I would love to come, but I have a prior engagement.
4. The proposal is interesting, but there are some concerns that need to be addressed first.
5. The presentation was well-prepared, but it lacked some important data.