Doing something very carefully and completely, without missing any parts. A thorough job checks all the details.
From Middle English 'thurgh' or 'thorow' meaning 'through, all the way.' It comes from Old English 'thurh,' the ancestor of modern 'through.' Over time, the sense shifted from physical movement through something to doing something completely in every part.
Thorough is basically 'through' turned into a personality trait—going all the way through a task. When teachers ask for a 'thorough explanation,' they’re asking you not to leave any gaps. The word quietly measures how deep your effort goes, not just whether you finished.
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