Late 16th century: formed irregularly from Latin tenuis 'thin' + -ous.

• thin from Old English:

  The Old English word thin shares an ancient root with Latin tenuis ‘thin, fine, shallow’, the source of extenuate (mid 16th century) and tenuous (late 16th century). An action which is unimportant in itself, but likely to lead to more serious developments is sometimes described as the thin end of the wedge. The idea here is of something being levered open by the insertion of the edge of a wedge into a narrow crack to widen the opening so that the thicker part can also pass through. The thin red line used to be a name for the British army, in reference to the traditional scarlet uniform. The phrase first occurs in The Times of 24 January 1855, reporting a debate about the distribution of medals for the Crimean War in the House of Lords at which the Earl of Ellenborough who spoke of ‘the services of that “thin red line” which had met and routed the Russian cavalry.’ It has now become so much part of our language that the colour may be altered to change the meaning—the thin blue line can mean the police force.