It means "head down in intense labour."
I'm sure this phrase was originally used for physical labour. But it has also been used, metaphorically, to describe the act of studying or any other mentally or physically strenuous activity.
This is how I would describe Skippy eating after some heavy duty paper strip attacking. =D
The Skippy Ticker, BTW, has been posted, but I made it so that it won't show up in your FRIEND View. I want to keep them on record, but rather than annoy everyone with the same stupid thing every day, if anyone is interested, you know where to find my journal. =)
We have more pictures and AVIs of her playing, but they're on Brother-in-Law's machine. I'll upload them later this afternoon. And I can say that she is cute now. =)