I consider myself rather good at vocabulary, but I have no idea what a "warren" is...sure I could look it up but I find it odd that such an opaque reference is used in such a casual way.
The Google dataset might not be the best for measuring this. Names and abbreviations rank highly. I think Warren Buffet might help contribute to the high ranking of 'warren'.
A warren is where rabbits and/or other burrowing creatures live. aka rabbit hole, burrow. It's common enough in children's literature like Watership Down.