Definition of "humpbacked"

There was a little stream, or brook, never dry, flowing, now slow, now with torrential rapidity, for ever in its narrow ditch. Unsteadily a rustic bridge bestrode its dark waters, a rustic humpbacked bridge, in a state of extreme dilapidation.

1953, Samuel Beckett, Watt, 1st American edition, New York, N.Y.: Grove Press, published 1959