do something on spec
1. Having the potential for profit but also the risk of financial loss. He made a fortune building houses on spec in the early 2000s, but when the economy crashed, he lost everything.
2. According to exact specifications. If you need any computers built on spec, they're the team to do it.
3. To attempt to do something without obtaining a ticket or any such thing in advance. Primarily heard in UK. You can't go to that play on spec—it's too popular, and you'll never get in.
do something on ˈspec(British English, informal) go somewhere without a ticket, an appointment, etc. in the hope that you will be able to get something you want: That restaurant’s very popular. You’ll never get a table if you just turn up on spec.
Spec is short for speculation (= doing something without knowing all the facts).