Stale

1 a of food : no longer good or appealing : no longer fresh

stale food

stale bread

1 b : not clean, clear, or pure : having an unpleasant taste or smell

stale air/water

a room filled with stale smoke

2 : not interesting or new : boring or unoriginal

stale news/jokes

Their relationship went stale.  [=became dull or predictable]

3 : not as strong, effective, or energetic as before

She felt stale  [=bored]  in her job.