A childish, angry overreaction to a negative situation or outcome; a display of bad temper, likened to a temper tantrum of a child. "Dummy" here refers to the British term for the device used to soothe babies (called a pacifier in the US or a soother elsewhere), which they may spit out when in the middle of a tantrum. Primarily heard in UK, Australia. John had a dummy spit when I told him I'd sold his old comic book collection.