What does "Spit the dummy" mean?
It means losing your temper, often in a sudden or childish way. A dummy is a baby pacifier, so the picture is someone chucking a fit.
The saying in a sentence
The saying
Spit the dummy
Example
He spat the dummy when his team lost.
When you use it
Use it to describe someone snapping over a small thing, usually as a joke between mates.
When you do not
Do not use it about someone who is genuinely distressed. It makes the reaction sound babyish.