1. To exit some event or activity and be replaced by someone else. ("Sub" is an abbreviation of "substitute.") The star quarterback is subbing out after sustaining an injury to his ankle.
2. To replace someone or something with someone or something else. In this usage, a noun or pronoun can be used between "sub" and out." With a healthy 30-point lead, the coach subbed the star quarterback for the rookie to give him some experience on the field. Don't believe anyone who tries to convince you that you can sub butter or oil out with low-fat cooking sprays without affecting the taste of your food.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.