Definition of "lumbago"

Sir Oswald Feiling winced as he turned to go home. He had felt a warning twinge of lumbago.

1935, Francis Beeding [pseudonym; John Palmer], chapter VII, in The Norwich Victims, London: Arcturus Publishing, published 2013, section 2

We may imagine an athletics coach with a scientific training researching into the physiology and the psychology of runners. [...] He finds out the effects of fatigue, of alcohol, of tobacco, of lumbago and of depression upon their performances.

1953, Gilbert Ryle, “Dilemma VII: Perception”, in Dilemmas: The Tarner Lectures, 1953, Cambridge, Cambridgeshire, New York, N.Y.: Cambridge University Press, published 1954 (1987 printing), page 105