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.