The ‘Lord Granard’ mentioned in quot. 1802 was George Forbes (1760-1837), sixth Earl and first Baron Granard.
He waddles on dragging his boots along in a way that would make a pig laugh. As Lord Granard says, a pig may whistle though he has a bad mouth for it.
[1802 M. Edgeworth Letter 19 Oct in Maria Edgeworth in France & Switzerland (1979) 10]
‘I dare say the Spanish sounds very singular to your ear.’ ‘Ay, sir; it puts me in mind o' an auld saying o' my faither the piper. “A soo may whussle, but its mouth is no made for't.”’
[1846 J. Grant Romance of War I. xii.]
Ye say he has the speech o' a guid Christian? Weel-a-weel, a soo may whistle, though it has an ill mouth for it.
[1927 J. Buchan Witch Wood xvii.]
Related to: possibility and impossibility
Bibliography of major proverb collections and works cited from modern editions is available here.