Down the Clyde

"Do you think I came down the Clyde on a biscuit?" mean? I think it's Scottish.

Although I am not familiar with this phrase, I believe it means 'do you think I am an idiot'.

It is like the Irish phrase (with which I am familiar) 'do you think I just floated down the Liffey on a lily pad?'

The essence being that the speaker is being treated like a new-comer to the big city: as an innocent abroad.

DFG