"It did nothing but make us ridiculous."
It means : "It did nothing but It did make us ridiculous."
Here, 'but' seems to be a conjunction.
But I know, 'but' is a preposition (meaning 'except') in the phrase 'nothing but'!
Again, if 'but' is a preposition, the verb following it should be in -ing from. But here the verb is in the base from - 'make'!
Then, what is the parts of speech of 'but' in the phrase "nothing but"?