Tá Mac Aoidh Bhuí ann agus oiche aréir léigh mé faoi cheann eile - Ó Fearghail Buí.
An bhfuil a fhios agaibh cén bunús a bhí leis an focal jBuí.

Does anybody know the origin of the word Buí in Irish surnames? I wonder was it to do with colour or if it has another meaning.

Before people had surnames, since there were many common given (first) names, adjectives were often appended to names to distinguish people from one another. The word buí can mean yellow, golden or tan, depending on the context (color terms can vary a lot from culture to culture), and it can even mean orange in some circumstances (an Orangeman is a fear buí). So, the person referred to as buí could have been a blond (much as the famous Owen Roe [Rua] O'Neill was presumably red-headed), or perhaps he was a very tanned person.

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I'm not a native (or entirely fluent) speaker, so be sure to wait for confirmations/corrections, especially for tattoos.

or just had jaundice....

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Go raibh maith agaibh.