Mia is obsessed with shoes and socks, so I taught her the signs for both so she would have an outlet for expressing her love for various forms of footwear. To make the sign for socks, you stick out just the index finger of each hand and then rub the sides of those fingers and the sides of your fists together. To make the sign for shoes, you make a tight fist with both hands and then knock the sides of your clenched fists together a few times.
Mia tends to take the signs we teach her and adapt them into her own, simpler or easier to perform versions. With shoes and socks, she seems to have decided that knowing two signs for these closely-related items is a waste of time, so she has combined the two signs into a single sign that she uses for both items.
When Mia wants to refer to either shoes of socks, she sticks out the index finger on one hand, makes a fist with the other, and jabs the fist with her finger a few times, more often than not making a rather juvenile but still incredibly rude gesture. (Don't get it? Try it yourself... one finger, one fist, now poke a few times.... ah, there, now you see.)
I've been a bit worried that Mia was going to learn to say "fuck" and run around town offending little old ladies - turns out, she doesn't even need to learn to speak to do that.