Excessive foot sweating causes socks to become extremely saturated - when footwear is removed the socks parody slugs in both appearance and evidence of location (a slug trail).
Usually following intense exercise in which participants are wearing tight fitting footwear.
Robert: Chea, that was an intense practice!
Milo: Yeah man, my feet were sweating so bad back there. Woah! Get a load of my slugsocks!