thorough [OE] Old English thuruh was an alteration of thurh ‘through’, and the two forms were both originally used for through. The adjective ‘carried out in every detail’ dates from the late 15th century, a period when it also meant ‘going or extending through something’ surviving in Late Middle English thoroughfare (literally ‘a track going through’), and familiar from Shakespeare’s ‘Over hill, over dale, Thorough bush, thorough briar’ in A Midsummer Night’s Dream.