Dirty weekend - Idioms by The Free Dictionary
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A secret getaway in which two unmarried people leave town to have sex. Primarily heard in UK, Australia. When Dana and Harry both skipped work on Saturday, we knew they had gone away for a dirty weekend.
a ˌdirty weekˈend (British English, humorous) a weekend spent away from home in order to have sex, usually with somebody who is not your usual partner: They went away for a dirty weekend in Brighton.