How do the principal 2021 meanings of "but" relate, if any, to its original meaning of "outside"? E.g. how does "no more than; only" appertain to "outside"?
- Used to introduce a phrase or clause contrasting with what has already been mentioned.
- [with negative or in questions] Used to indicate the impossibility of anything other than what is being stated.
- [archaic with negative] Without it being the case that.
- No more than; only.
But originally meant ‘outside’. It was a compound word formed in prehistoric West Germanic from *be (source of English by) and *ūtana (related to English out). This gave Old English būtan, which quickly developed in meaning from ‘outside’ to ‘without, except’, as in ‘all but me’ (the sense ‘outside’ survived longer in Scotland than elsewhere). The modern conjunctive use of but did not develop until the late 13th century.
Word Origins (2005 2e) by John Ayto. p 84 Left column.