The phrase "grim determination" means having a very strong and serious attitude to achieve or overcome something, even if the task is difficult or unpleasant. Full definition
I remember a lone tear and a feeling of grim determination that has never been satisfied. (dealspwn.com)
Cal and Niko aren't straightforward good guys; they are the Batmans of their world, surviving by grim determination and a steady aim. (goodreads.com)
You're serious about growing your company, but would replacing grim determination with a bit more levity actually be better for your business? (inc.com)