One of the mysteries of the English language finally explained.
1A platform or other support erected on a roof to facilitate repair, painting, etc.
2A smoke vent of a chimney; (also) an airtight fitting attached to the roof and joining the internal and external sections of a vent pipe.
3A pole supporting the roof of a tent. rare.
Late 19th century; earliest use found in The San Francisco Chronicle. From roof + Jack.
roof jack/ˈruːf dʒak//ˈrʊf dʒak/
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