Other forms: letting sleeping dogs lie; lets sleeping dogs lie
To let sleeping dogs lie is to avoid stirring up old problems or issues. Why risk causing a whole new argument with a friend by dredging up an old one? Sometimes it's better to just let sleeping dogs lie.
The idiom let sleeping dogs lie is based on the literal idea of not waking a dog that might turn out to be aggressive or grumpy from being startled awake. Imagine you’re walking past a place with a guard dog that's fast asleep in the yard. If you wake it up, there's likely to be a lot of barking and unnecessary hullabaloo. So — shh. That’s the idea behind the expression let sleeping dogs lie. It’s the lie that means "be lying down," not the lie that means "say something untrue."