The Spanish word 'mas' (meaning 'but') comes from the Latin word 'magis' meaning 'more'. It evolved through Old Spanish 'maes' to modern Spanish 'más' (meaning 'more'), and then underwent a semantic shift to become 'mas' (meaning 'but'). This semantic evolution from 'more' to 'but' likely occurred as the word was used in contrasting contexts, where 'more' could imply an addition or contrast to what was previously stated.

The most obvious related Spanish word is 'más' meaning 'more', which is actually much more commonly used than 'mas'. While 'más' retained the original Latin meaning of 'more' (as in 'más grande' = 'bigger', literally 'more big'), 'mas' developed into a conjunction meaning 'but'. It's worth noting that 'mas' meaning 'but' is somewhat formal or literary in modern Spanish, with 'pero' being the more common word for 'but' in everyday speech.

While there aren't any direct English cognates from Latin 'magis', English speakers might recognize the root in words like 'master' and 'magistrate', which come from related Latin words sharing the idea of 'greater' or 'superior'. The comparative suffix '-most' (as in 'uppermost', 'foremost') is also distantly related, coming from the same Proto-Indo-European root that gave rise to Latin 'magis'.