counter for pairs of footwear
Used for counting pairs of items worn on the feet, such as socks, shoes, and tabi. Not used for single items.
新しい靴下を二足買った。
I bought two pairs of new socks.
この靴は一足いくらですか。
How much is one pair of these shoes?
The noun 足 (あし) means 'foot' or 'leg', while 足 (そく) is a counter for pairs of footwear.
The counter そく is derived from the on'yomi of the kanji 足, which originally means 'foot' or 'leg'. The use as a counter for pairs of footwear is a natural extension.