1344: but Aug 16, 2018

The conjunction 'but' is now used is so many ways that to define it any other way than as simply 'introducing clauses to be contrastive'. The word originated in Old English as 'be-ūtan' meaning 'without' or 'except', and it took a while for it to be used to introduce new clauses with the frequency it can now. The line in Shakespeare's Macbeth that ends with "that but this blow might be the be-all and the end-all here, but here, upon this bank and shoal of time, we’d jump the life to come" contains the two usages in question, the former sounding far more archaic and perhaps rarer than the latter, but in its early-days, that was the more common sense.
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