Scottish National Dictionary (1700–)
ROUST, n. Also roost, rust; rost; roast. A turbulent stretch of sea caused by a strong current in a restricted passage or by the meeting of conflicting currents, specif. in the seas between Sh. and Ork. between the Atlantic Ocean and the North Sea (Ork. 1808 Jam.; Sh., Ork. 1866 Edm. Gl.; I.Sc., Cai. 1967). [Sh. rust; Ork. rɔst]
Ork. 1701 J. Brand Descr. Ork. 73:
Impetuous Tides which they call Rousts, caused by the strong current of a Tide meeting with a narrow passage. Sh. 1775 J. Fea New Method of Fishing (1884) 147:
They dare not venture into the Roast, or main of the Tide, for fear of being swallowed up. I.Sc. 1821 Scott Pirate i.:
This lofty promontory is constantly exposed to the current of a strong and furious tide, which, setting in betwixt the Orkney and Zetland Islands, and running with force only inferior to that of the Pentland Firth, takes its name from the headland we have mentioned, and is called the Roost of Sumburgh; roost being the phrase assigned in those isles to currents of this description. Sc. 1886 Stevenson Kidnapped xiii.:
The roost or tide race which had carried me away so fast and tumbled me about so cruelly. Ork. c.1912 J. Omond 80 Years Ago 5:
We all now associate Eynhallow with the foaming roosts on either side . . . from Evie, across the stormy roost, might be heard the call to matin or to evensong. Sh. 1949 Scots Mag. (Nov.) 130:
The tide-races, of which there are many in Shetland, are roosts or horkels.
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"Roust n.". Dictionary of the Scots Language. 2004. Scottish Language Dictionaries Ltd. Accessed 15 Dec 2017 <http://www.dsl.ac.uk/entry/snd/roust>
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