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.
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   2. A kind of weir for catching fish. [Eng.] --Carew.
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The Collaborative International Dictionary of English v.0.48:

Frith \Frith\, n. [OE. frith peace, protection, land inclosed
   for hunting, park, forest, AS. fri[eth] peace; akin to
   freno[eth] peace, protection, asylum, G. friede peace, Icel.
   fri[eth]r, and from the root of E. free, friend. See Free,
   a., and cf. Affray, Defray.]
   1. A forest; a woody place. [Obs.] --Drayton.
      [1913 Webster]

   2. A small field taken out of a common, by inclosing it; an
      inclosure. [Obs.] --Sir J. Wynne.
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