The Collaborative International Dictionary of English v.0.48:

Frith \Frith\ (fr[i^]th), n. [OE. firth, Icel. fj["o]r[eth]r;
   akin to Sw. fj[aum]rd, Dan. fiord, E. ford. [root]78. See
   Ford, n., and cf. Firth, Fiord, Fret a frith, Port
   a harbor.]
   [1913 Webster]
   1. (Geog.) A narrow arm of the sea; an estuary; the opening
      of a river into the sea; as, the Frith of Forth. Also
      called firth.
      [1913 Webster]

   2. A kind of weir for catching fish. [Eng.] --Carew.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Firth \Firth\ (f[~e]rth), n. [Scot. See Frith.] (Geog.)
   An arm of the sea; a frith.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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: Firth, J. R. Firth, John Rupert
         Firth]
    2: a long narrow estuary (especially in Scotland)