An ice sock is a sock filled with ice, used by cyclists participating in brevets (long-distance rides) to avoid overheating. A white athletic sock is filled with ice, which the cyclist drapes around his/her neck. This is a low-cost and effective method to keep the cyclist's temperature in check and prevent overheating.
Other mentions of ice socks include cold weather sock products to offer feet protection against cold weather, as well as ice melt socks to prevent ice dams on roofs.
References
Ice sock Wikipedia(Text) CC BY-SA