Source : Webster's Revised Unabridged Dictionary (1913)

Firth \Firth\, n. [Scot. See {Frith}.] (geog.)
   An arm of the sea; a frith.

Source : WordNet®

Firth
     n 1: English linguist who contributed to linguistic semantics and
          to prosodic phonology and who was noted for his
          insistence on studying both sound and meaning in context
          (1890-1960) [syn: {J. R. Firth}, {John Rupert Firth}]
     2: a long narrow estuary (especially in Scotland)