To 'chow bears' is to become very physically tired when exercising. The phrase came about due to the feeling of 'having a bear on your back.' Obviously this would slow you down somewhat and make you feel a bit exhausted. To be 'chowing bears' would be to start feeling this sensation.
Often occurs when performing cardiovascular exercise the day after a big night, or when exercising after a few fags or pints.
"Wow bro, I'm chowing serious bears after that run!"
"Ya no me too, I'm chowing bears after I chowed that gap in the line. Mondo. Think I'm gonna need to be substituted soon."
"Coach, I chowed bears, please get me off the field!"
"Dude, you look fucked! Chow bears?"
- "Big time bro"