Definition:
Thumb Voting is a voting format to agree upon alternatives after they are discussed extensively. To create the alternative proposals for group decisions, there are other formats.
Thumb Voting is a simple version of the group decision Five Finger Voting or Five-to-Fold facilitation for decision-making. A group has to decide in a very short time on a proposal and possible alternatives to solve the issue.
Further Reading:
Book: Applying UML and Patterns by Craig Larman.