Ponder, think about, as in She mulled over the offer for some time and then turned it down. [Late 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
How to use mull over in a sentence
A group of friends gathers to mull over what to do with a day off.
However, I have taken a step back mull over it to think about what Conard is saying, even if he is being gruff about it.
But I've got to mull over it a little before it would make sense.Dave Dawson with the Commandos | R. Sidney Bowen
Couldn't you come now and mull over the alterations which you are going to make in your MS, and make them after you go back?The Letters Of Mark Twain, Volume 3, 1876-1885 | Mark Twain (Samuel Clemens)
Never mind all that, Zizi said, impatiently, it wont get us anywhere to mull over that.The Luminous Face | Carolyn Wells