The story
According to some, the phrase originated at a time when English pubs closed in the afternoon as a term for customers who were waiting or arrived soon after the pub re-opened in the evening. Alternatively, the phrase is said to originate in the theatre world, where it described a facility available to early-arriving members of the audience, who were allowed to avoid the crush and choose the best seats in return for paying a small premium.
Etymology from Wiktionary, CC BY-SA 4.0.