I am pretty much a monoglot but it was explained to me as, THIN PORRIDGE, THICK PORRIDGE.
This is a waulking the tweed 'song'. Imagine rows of women 'manipulating' a bolt of cloth all chanting that 'song' to keep in time.



http://www.youtube.com/watch?v=fFoO6A7oRH0&feature=related
Not the right song and the bolt would be wet (the waulking/manipulating would 'close' the weave of the cloth).