Lazy Cow!

I'm not sure why  - well I'm excusing myself on the grounds of IBS and the broken leg!!!! Hopefully I'll find some VaVaVoom soon!
I couldn't find where the term comes from - it's not a barnyard idiom. Plus I don't think cows are lazy - they may lie down a lot but then grass is hard to digest and they have to pass it through their  four stomachs - hardly lazy!