When one changes lanes on a freeway or highway without hitting any of the reflecting bumps in the middle of the road.

A merge-swish can also be applied when passing someone on a two lane road, provided you do not hit any of the reflective bumps in the middle of said road...or the oncoming traffic.
Bill Blimey: "Man, I just changed lanes three times and totally merge-swished every time. It was nothing but line!"

Tom Toppover: "That's weak sauce. You should have seen the merge-swish I pulled off last night while passing that big rig."
by Stopping Stan October 13, 2011
Get the merge-swish mug.