Primarily boisterous saying, knocking back drink (7,2)
I believe the answer is:
bottoms up
'primarily boisterous saying knocking back drink' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'primarily boisterous saying knocking back drink' is the wordplay.
'primarily' indicates taking the first letters.
'saying' becomes 'motto' (motto is a kind of saying).
'knocking back' says the letters should be written in reverse.
'drink' becomes 'sup' (to sup is to drink).
The first letter of 'boisterous' is 'b'.
'motto' written backwards gives 'ottom'.
'b'+'ottom'+'sup'='BOTTOMS UP'
Can you help me to learn more?
(Other definitions for bottoms up that I've seen before include "Cheers!" , "Toast" , "Down the hatch!" , "down it goes!" , "Your good health!" .)