An underplayed aspect of Scots song is its mouth music, the lowland cousin to the puirt a beul of the Gaidhealtachd. Similarly, largely meaningless or comical lyrics are used to underpin the rhythm of a particular dance tune. This is separate from diddling, which uses vocables to sing a dance tune.

Here are some examples from the Kist o Riches website: 

The Smith's a Gallant Fireman sung by Willie Mathieson of Ellon

Mouth music medley by Jean Redpath

The fast nature of the tunes is also an excellent medium for masking rather risqué lyrics, as in both Tail Toddle and Brose and Butter. 
 
Tail Toddle sung by Hamish Henderson on the Kist o Riches website

Brose and Butter by the group Ossian: 

In some cases, Scots mouth music is macaronic (uses a mixture of languages) - such as Lizzie Higgins' text of Macaphee Turn the Cattle (at the Kist o Riches website), to the dance tune of Mrs MacLeod of Raasay. The Gaelic text is Mac a' Phì cnag-shùileach, Ceann Loch a' Feòrain, "peg-eye MacPhee at the head of Loch Feorain".

Thanks to Margaret Stewart for assistance with the Gaelic of Macaphee.