not fresh; vapid or flat, as beverages; dry or hardened, as bread.

musty; stagnant

stale air.

having lost novelty or interest; hackneyed; trite

a stale joke.

having lost freshness, vigor, quick intelligence, initiative, or the like, as from overstrain, boredom, or surfeit

He had grown stale on the job and needed a long vacation.

Law . having lost force or effectiveness through absence of action, as a claim.

to make or become stale.