The thou (abbreviation th) or mil (Chiefly United States) is a unit of length in the Imperial System and the United States Customary Units. It is equal to 1⁄1,000 of an inch. This is not to be confused with the old form of "you".
Thou comes from the Germanic languages, a shortening of thousandth.
Mil comes from the Romance languages, a shortening of mille.
The thou was introduced in 1844 by Joseph Whitworth as a substitute to traditional fractions such as 1⁄64 1⁄32 1⁄16 1⁄8 which cannot be defined simply in hundredths unlike 1⁄4 (0.25) or 1⁄2 (0.5). Traditional fractions continued to be used, as they were a substantial part of other units of length.