Right, here you are:

THE MHM SONG
(Trad)

Ye'll have heard o' the de'il as he wandered thro' Beith
With one wife in each oxter and one in his teeth
When someone cried out, Will ye take mine in the morn
But he waggit his tail and he cockit his horn
And he only said, M-hm
That wee bit word M-hm
He'd such a big moothfu' he couldnae say, Aye

When I was a laddie lang syne at the school
The Maister he cried me a dunce and a fool
But for a' that he said I could ne'er understand
Except when he roared, Jimmy haud oot yer hand
But I only said, M-hm
I grim'ced and said, M-hm
I wasnae o'er proud but o'er dour tae say, Aye

One day a queer word as langnebbed as himsel'
He vowed he would thrash me if I couldnae spell
Says I, Mister Quill, wi' a kind o' a quiver
I'll spell ye yer word if you'll spell me anither
Let me hear you spell M-hm
That wee bit word M-hm
That auld Scots word M-hm, ye ken it means aye

Had ye seen how he scoured and he scairted his pate
And he cried out, Ye villain, come oot o' yer seat
Get oot o' ma gate, ye're the plague o' the school
And the de'il on me kens if you're most rogue or fool
But I only said, M-hm
I smiled and said, M-hm
That auld Scots word M-hm, ye ken it means aye

Now M-hm's no big but it's gie hard to spell
If ye saw it in print would ye ken it yersel'
So if yer o'er lazy tae open yer maw
Just keep yer mouth shut and say nothing at a'
Never say, M-hm
That wee bit word M-hm
That auld Scots word M-hm, ye ken it means aye

(From the Corries Songbook)
Not surprisingly, I have not a scrap of further info on the song.