Green’s Dictionary of Slang

junket n.

SE in slang uses

In compounds

junket-brain (n.) (also junket-head) [SE junket, whey, + SE brain/head, thus ‘soft brain’]

(Aus.) a general term of abuse.

1955		D. Niland Shiralee 102: Listen, junket-brain, it’s either Kelly or nothing.
1957		D. Niland Call Me When the Cross Turns Over (1958) 114: Give these junket-heads in here something nice to look at and they don’t appreciate it.