bobby sox
1943, from diminutive of bob, n.2 + sox. So called because they are "shortened" compared to knee-socks. Derivative bobby-soxer first attested 1944.
Months ago colored bobby sox folded at the top were decreed, not by anyone or any group but, as usual, by a sudden mysterious and universal acceptance of the new idea. Now no teen-ager dares wear anything but pure white socks without a fold. ["Life" magazine, Dec. 11, 1944]