ˌtrick or ˈtreat verb

1. go trick or treating if children go trick or treating, they dress in costumes and go from house to house on Halloween saying ‘trick or treat’ in order to get sweets

2. the words that children say when they go trick or treating, to say that they will play a trick on someone if they are not given a treat (=sweet)

[TahlilGaran] Dictionary of Contemporary English ▲

trick or treat
n. The custom of going from house to house on Halloween asking for small gifts and playing tricks on people who refuse to give.
When Mrs. Jones answered the doorbell, the children yelled "Trick or treat." Mrs. Jones gave them all some candy.
On Halloween Bill and Tom went out playing trick or treat.

[TahlilGaran] English Idioms Dictionary ▲