Of a vehicle, for its rear to slide erratically from side to side on the road (likened to a fish's tail moving back and forth). The roads are pretty icy today, and I saw a couple of cars fishtail coming around that corner.
fishtail
in. [for the rear of a car] to whip back and forth like a fish moving its tail. The caddy fishtailed on the curb and almost spun around.