let the grass grow under (one's) feet
To be inactive; to do nothing or stand still. Often used in the negative as an imperative, such as "don't let the grass grow under your feet." I used to let the grass grow under my feet, and I missed out on a lot of opportunities. Don't let the grass grow under your feet, kid. Get to work!
let grass grow under one's feet
Fig. to do nothing; to stand still. Mary doesn't let the grass grow under her feet. She's always busy. Bob is too lazy. He's letting the grass grow under his feet.