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:

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.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "frith":
   arm, armlet, bay, bayou, belt, bight, boca, boscage, bosket, brake,
   canebrake, ceja, chamisal, chaparral, coppice, copse, copsewood,
   cove, covert, creek, estuary, euripus, fjord, gulf, gut, harbor,
   inlet, kyle, loch, motte, mouth, narrow, narrow seas, narrows,
   natural harbor, reach, road, roads, roadstead, sound, strait,
   straits, thicket, thickset, underbrush, undergrowth, undershrubs,
   underwood