Bears do what in the winter?!

Tonight one of the books we read to the twins (and Ethan) was Bear Stays Up For Christmas. Naturally the bear's friends had to wake him up so he didn't miss Christmas and then he fell asleep right after the celebration. So, I asked Ethan why the bear was so sleepy. He said that bears sleep through the winter. Then...

M: What's that called... when animals sleep through the winter?

E: Um... I know... um... (obviously trying hard to remember the word)

M: Hi... (giving him a hint of the first sound)

E: Hibrocobia! (shouting it out confidently and knowingly)

HUH??! I think I need to contact Webster's... it could be a new word! Seriously, we (and he) don't know where that came from, but we ALL couldn't stop laughing the rest of the night.