Geoff Alday talks to the creator of the ‘hamburger icon’:

You’ve done your homework and found the right guy. I designed that symbol many years ago as a “container” for contextual menu choices. It would be somewhat equivalent to the context menu we use today when clicking over objects with the right mouse button.

Its graphic design was meant to be very “road sign” simple, functionally memorable, and mimic the look of the resulting displayed menu list. With so few pixels to work with, it had to be very distinct, yet simple. I think we only had 16×16 pixels to render the image. (or possibly 13×13… can’t remember exactly).

Interesting inside joke… we used to tell potential users that the image was an “air vent” to keep the window cool. It usually got a chuckle, and made the mark much more memorable.
Air vent. Love it. I wouldn’t be surprised if there were people who actually believed they truly functioned as vents for windows.
Actually, I wouldn’t be surprised if there were people who believed that now.