piston slap on a 2000 suburu ?

It means the engine is pretty much worn out.

DA is correct. MOST early noughts 2.5i suffer from cold-running piston slap as their rings were speced loose enough to wobble until isothermal running ocurs at set temp. If doesn't completely quench after 20min be careful to not drive under WOT at low rpm to reduce stress. You'll probably be ok unless it still hammers at high rpm when hot, indicateing throwing a rod sometime fairly soon. Good luck.