stagnate

To cease motion, activity, or progress:

If the water stagnates, algae will grow.

To cease motion, activity, or progress:

Air stagnates in a closed room.

To cease motion, activity, or progress:

Ready-witted tenderness […] never stagnates in vain lamentations while there is any room for hope.

Listening to what others say, especially to what they teach, prevents our minds stagnating, thus promoting mental growth into old age.

Stagnant.