a sound like that of the wind, the wind itself, a draught. The ou is pronounced as oo and the ch is pronounced as it is in loch. Even if the word is spelt differently – and it clearly shows that Scots spelling is very variable – it retains this pronunciation. Some of these spellings, as well as senses are illustrated in these quotations:
Cauld blaws the nippin north wi’ angry sough (Robert Fergusson Poems 1773).
The wind was back a bit and a strong seuch coming up from the south (Naomi Mitchison & Denis Macintosh Men & Herring 1949).
November chill blaws loud wi angry sugh (Robert Burns The Cotter’s Saturday Night 1785).
Ye’re clear o’ the soogh o’ the door there (Alexander Hislop The Book of Scottish Anecdote 1876).
If you keep, haud, ca’ or mak a calm, quiet, sober or lown souch then you keep quiet or hold your tongue. Keeping a calm souch also means remaining unperturbed. By contrast to raise yer souch means to speak out or make a pronouncement. If you mutter something within yer ain souch it is said under your breath or sotto voce.
A highly descriptive use as a verb appears in John Buchan’s Poems, Scots and English from 1917:
The fiddles scrapit and atower the din
The “Floo’ers o’ Embro’” soughed oot on the win’.