1.
If you ~ a drink, especially an alcoholic one, you drink it quickly, and often in large amounts. (INFORMAL)
He was knocking back his 10th gin and tonic of the day...
She poured some vodka into a glass and knocked it back in two swallows.
PHRASAL VERB: V P n (not pron), V n P
2.
If an event, situation, or person knocks you back, they prevent you from progressing or achieving something. (mainly BRIT)
It seemed as though every time we got rolling something came along to knock us back...
That really knocked back any hope for further peace negotiations.
= set back
PHRASAL VERB: V n P, V P n (not pron)