A minute with an indeterminate duration. A metro minute could be 30 seconds or 30 minutes, and you don't know how long it lasted until it's actually over. Derived from the Washington Metro Area Transportation Authority (WMATA) metro train arrival times, which can never be taken at face value.
Jim: Man, it looks like I got to the station right on time. It says the next metro train will be here in 2 minutes.
Tim: You know those are metro minutes, right? Either the train has already pulled into the station and we missed it, or we're gonna be here for a while.
Sarah: Where the hell is Jessica? She said she was changing outfits and will be out in a minute.
Kara: She meant she'll be out in a metro minute. Just sit tight, it could be a while.