Green’s Dictionary of Slang

rocker n.1

[1910s] (Aus.) the mind, the brain.

In phrases

off one’s rocker (also off one’s rock) [SE rocker, a rocking-chair]

1. [late 19C+] crazy; occas. antithesis, on one’s rocker.

2. [1900s] (Aus.) drunk.

3. [1950s+] in fig. use, acting excessively (though not necessarily madly).