Off Piste
“Piste” is French for trail or track. So, “off piste” is a skiing term for backcountry skiiing, meaning to ski in areas that are not groomed slopes at ski resorts.
As an idiom, “off piste” could mean to do something unexpected or outside the ordinary.
Why I Looked It Up
I saw a billboard in the London Underground promoting “off piste” as a lifestyle choice. It’s an interesting angle, since it’s likely that a lot of non-skiers wouldn’t understand the term.