a pleasant situation or event that is not planned or intended.

We never planned to have a third child - it was a happy accident.

• be off the beaten track
• be off the blocks
• be off the hook
• be off the mark
• be off the starting blocks
• be off the wall
• be off trolley
• be off your chump
• be off your nut
• sit on arse
• sit on backside
• sit on hands
• sit on the fence
• sit on your arse
• sit on your backside
• sit on your hands
• sit through
• sit through something
• sit tight
• sitting duck
• sitting on top of the world
• sitting pretty
• sit up
• sit up and take notice
• sit well with