To have a childish overreaction or angry outburst to a negative situation or outcome; to act in a bad-tempered manner, likened to a temper tantrum of a child. "Dummy" here refers to a plastic teat used to soothe teething infants (also called a "pacifier" in the U.S. or a "soother" elsewhere), which they may spit when in the middle of a tantrum. Primarily heard in Australia. John spat the dummy when I told him I'd sold his old comic book collection. Don't go spitting out the dummy just because Cheryl got the promotion instead of you.