Hard physical or mental work, or to work hard at something (British spelling of 'labors').
From Latin 'labor' meaning toil or work, borrowed into Old French as 'labeur.' The word has meant difficult work since ancient times and became particularly associated with manual labor.
The phrase 'labour of love' exists because Latin speakers used 'labor' to describe work so difficult it was almost painful—yet we still use it for things we choose to work hard on because we love them, flipping the ancient meaning on its head.
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