lucky dip
1. noun A game in which a person (typically a child) pays to search through a container in the hopes of picking out a prize that cannot be seen. Primarily heard in UK. I remember playing the lucky dip when I was young. In those days, cheap little toys were hidden in barrels of sawdust, and you had to pick them out with your eyes closed!
2. noun Any process or method in which the outcome is chosen or selected at random or without certainty. Primarily heard in UK. Buying a car online that's worth your money can be something of a lucky dip.
3. adjective (hyphenated and used before a noun) Of or having a process of random selection. Primarily heard in UK. To make room selection fair for all incoming students, the university decided to implement a lucky-dip system for room assignments.