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All day, every day. Short for "twenty-four hours a day, seven days a week." Another hyphen can be used between "four" and "seven" if used as a modifier before a noun. Often written as 24/7. You know you can call me twenty-four seven if you ever need anything, OK? It's the only shop in the entire city that's open 24/7. We offer twenty-four-seven customer support for anyone whose credit card is lost, stolen, or locked.
See also: seven