1.
If you wash something, especially food, down with a drink, you drink the drink after eating the food, especially to make the food easier to swallow or digest.
He took two aspirin immediately and washed them down with three cups of water.
PHRASAL VERB : V n P , also V P n (not pron)
2.
If you wash down an object, you wash it all, from top to bottom.
The prisoner started to wash down the walls of his cell.
PHRASAL VERB : V P n (not pron) , also V n P