1
general::
Phrase(s): knock someone back (an amount of money)
to cost an amount of money. • That trip to Spain knocked me back almost $3,500. • How much did that new furniture knock us back?, Phrase(s): knock something back
Sl. to drink down a drink of something, especially something alcoholic. (See also knock back a drink.) • I don’t see how he can knock that stuff back. • John knocked back two beers in ten minutes.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Also, knock it back. Gulp down an alcoholic beverage, as in He knocked back glass after glass of wine, or I hear you've been knocking it back a bit. [First half of 1900s]
American Heritage Idioms