Pertaining to a mixture of two or more languages. The language spoken/written in Pepé le Pew cartoons is a good example ("pourquois runnez-vous?"). It's common in poetry and songs too.
Believe it or not, the word actually does come from "macaroni." I'll have to double-check on the history of how that happened.
Here's an example of a macaronic Welsh song:
Ye lads all thro' the country
Gwrandewch ar hyn y stori (listen to this story)
You better go dros ben a graig (over the top of a cliff)
Than go with gwraig i'r gwely (wife to bed)
And so on...