nose to the grindstone, keep one's
Stay hard at work, as in We expect John to get good grades again, since he really keeps his nose to the grindstone. This expression, first recorded in 1539, alludes to a tool that must be sharpened by being held to a grindstone.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.