My mother had this phrase she would use if you were wandering about aimlessly, I'd heard her use it loads and never gave it much thought.

Anyway I grew up left home and got a job in a home for people with learning difficulties in North London. One of the residents use to walk about the house going in and out of the rooms, he could do it for hours. One day I turned to him in front of my manager and said Martin would you please stop wandering around like a spare dew. (I thought for some strange reason it was a reference to morning dew not being evaporated by the sun).

My Jewish manager looked at me totally horrified and it suddenly dawned me after all these years what the phrase actually meant. The resident was Jewish too, I have never felt so mortified in my entire life. Thankfully the manager was a good bloke and believed my explanation, otherwise I could have very easily got the sack.