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1.
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
by Bob Growler August 06, 2007
 
2.
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"
by Zeromus255 November 15, 2011
 
3.
Taking a left hand turn from the center lane (or lane to the right of the left turn lane) so as to avoid the traffic in the turning lane.
"The line to take a left sucked, so i said fuck it and took a boston left.
by gobostongo February 17, 2009