let it rip
Also, let her rip. Go ahead, proceed unchecked. For example, Once you get the tractor started, let it rip. The use of her in the variant comes from a tradition of referring to vehicles as feminine. [Mid-1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.