A grunt is a low noise people make when they're tired or hurting.
When someone says "I'm going for a run," and then after a few minutes they stop and say "Ugh, I think I've got a bit of a grunt left in me!" they mean their legs are really sore and they can only make this low, gruff sound when they breathe out. It's not just for tiredness, though – you might also hear a grunt when someone lifts something heavy or tries to do something difficult. For example, if you're trying to move a big box across the room and strain your back, that could be a grunt too!
A short snorting sound, often to show disapproval, or used as a reply when one is reluctant to speak.
(of a person) To make a grunt or grunts.
