Energetically, forcefully, loudly. For example, This is a soft passage—the horns shouldn't come in like gangbusters. This expression alludes to a popular radio series entitled Gangbusters, which featured explosive sound effects, such as gunfire and sirens, at the beginning of each episode. [Slang; late 1930s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.