A left hand turn that is started far too early and made at approximately 45 degrees instead of 90 degrees. Used primarily when trying to make a turn ahead of an oncoming car, often to cut in front of a long line of oncoming cars.
That dude in the Monte Carlo just banged a harsh Boston Left
If you are the first person in line in a left-turn lane, a Boston Left is the act of pulling your car into the middle of the intersection and waiting until you can turn, whether or not the light has turned red. Its a way to "force" yourself through a light, because the opposing flow of traffic will be unable to proceed with you in the way.
"I dont care if the light is red or not, Im not waiting another 5 minutes to turn- Im pulling a Boston Left"