Bite off more than you can chew
Definition 1 of 1
Pro Tip 1/3Metaphorical Use
This phrase is used metaphorically to describe overcommitting, not literally chewing. 
Getting into three sports at once, she's bitten off more than she can chew.
Pro Tip 2/3Negative Connotation
It suggests a negative outcome from taking on too much. 
He bit off more than he could chew and missed all his project deadlines.
Pro Tip 3/3Use with Caution
Avoid using this phrase in formal or professional contexts as it can seem informal. 
In the meeting, I didn't say he'd bitten off more than he could chew.