'auld' originates from Old English, specifically the word 'eald' (meaning 'old'), which passed into Middle English as 'ald'/'auld' and is used in Scots as 'auld'. 'auldest' is the superlative formed from that Scots adjective.

'auld' changed from Old English 'eald' to Middle English forms 'ald'/'auld' and was retained in Scots as 'auld'; the superlative formation produced 'auldest', paralleling English 'old' → 'oldest'.

Initially it meant 'old' (as in age); over time in Scots it remained as the dialect form 'auld', and 'auldest' retained the meaning 'most old' or 'oldest'.