やりそう (yarisou)

やる (yaru) means do, same as する。
そう (sou), in this situation, means something akin to "seems like".

So やりそう means "It seems like you'd do that,"  or "It seems like something you'd do.

To illustrate, there was a class in which one of my students said this, and it made me laugh uncontrollably.

We were discussing an article about child discipline.  I asked everyone how they felt about corporal punishment.  After some small group talking time, we went around the room for people's views.  One member of the class said, "I think it is okay, if it is on a soft part of the body."  I think we all understood what she meant: spanking a child's bottom was okay, but hitting him/her, bruising, etc. was not.  Striking bone onto bone was not okay.  (I would like to mention that this student is a very nice person, having known her for two years, and I would bet all the money I have in this world that she wouldn't abuse a child.)

But, just as a joke, I asked her, "So this is okay?"  and I mimed someone hitting another person in the stomach.

The student immediately shook her head no.  Her friend, sitting next to her, shouted out, "Yarisou!", meaning that she thought the student would do something like that.  Punch her kid in the gut.  Of course, the friend was kidding, and we all shared a nice laugh.