Mexican jumping bean

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A type of seed of a shrub of the genus Sebastiania in which a larva has hatched from an egg laid there by a moth, the movement of the larva causing the seed to move around.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun seed of Mexican shrubs of the genus Sebastiana containing the larva of a moth whose movements cause the bean to jerk or tumble