To eat large quantities of carbohydrates, as one would typically do in preparation for running a marathon. "Carbo" is short for "carbohydrate." You better start carbo loading now if you want to make it through the race on Saturday.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
Consume a large amount of carbohydrate food, as in Karen began carbo loading three days before the road race. This term, a clipping of "carbohydrate loading," originated among marathon runners, who were advised to build up their strength before a race by eating quantities of foods like spaghetti. [1970s]
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.