Why Do They Call Them the "Terrible Twos"?

I'm not asking this question to wonder about the "terrible" part, but really wondering where the "twos" came from! Everyone I know says that their child's "terrible twos" started much before their 2nd birthday. Sophie, apparently, is no exception.

Last night, our naughty little girl received two time outs. She seems to be testing the boundaries of the word "No." She started throwing coasters on the ground last night, and when I told her to pick it up, she threw a second one down. When I told her "no" and to pick up the two on the ground, she looked at me (without breaking eye contact) and threw a third one on the ground. She got a little time out for that (and Spongebob was turned off). She was more upset about Spongebob disappearing than sitting in the corner, but when she was done, she came over and picked up the coasters.

You'd think that would be enough naughtiness for one night, but no. While eating dinner, she started kicking her highchair with her shoes. I told her to stop, and she didn't. I told her to stop or she would get a time out. She looked at me (again without breaking eye contact), smiled...and kicked again. This time she was very angry about time out because I removed her from her dinner. She cried the whole time...but didn't kick her highchair anymore!

I can't wait until she turns two to see if this is just a warm up to the real "terrible twos"!