It All Started on a Telephone Pole
The bicycle messenger bag, as we know it today, has its roots in New York City in the 1950s. Legend has it that Globe Canvas, owned and operated by the late Frank Demartini, began manufacturing heavy-duty canvas shoulder bags for telephone linemen. These bags were designed for easy access to tools and supplies while the linemen were suspended on telephone poles.
In the late 1970s, bicycle messengers in New York began using bags from Globe Canvas for their daily deliveries. In response to increasing motor vehicle congestion in rapidly growing cities, the bicycle messenger profession grew. The bicycle messenger community developed an identifiable subculture.
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I got the history from their site.