I've also heard it's where 'You dig?' Comes from for understanding. Of course, that might've been from Cassidy... whose work is awful, really.
The American phrase becomes more obvious if you say "An dtuigean tú, Daideo" "Do you understand, Grandad?"
I have also heard that "smashing" an English slang word comes from "is maith sin" I think that's interesting.
The only sentance EVERYONE in school needed for the irish orals "ní thuigim an cheist!" And by jaysus it got used!!
As @galaxyrocker mentioned it is rumoured to be from the Irish phrase. Blindboy boatclub claims on his rubber bandits podcast that when the Irish and blacks were living in close quarters in the US cities in the 19th and early 20th century, the blacks picked up the cupla focal, and that that is where the term 'you dig' comes from!
From the course notes and tips I don't follow which rule causes the eclipsis above