Green’s Dictionary of Slang

stiffen v.1

[ext. stiff v.3 (1)]

1. [late 19C] (Aus., also stiffen out) to die.

2. [late 19C] (Aus.) in fig. use, to punish.

3. [late 19C–1900s] to kill, to murder; lit. or fig.

4. [late 19C+] (Aus., also stiffen out) to knock someone unconscious, thus stiffener, that which knocks one out (lit. or fig.).

5. [late 19C-1900s] (Aus.) in fig. use, to ruin, to undermine, to ‘put paid to’; to interfere with a racehorse’s performance; thus n. stiffener, one who pursues such activties.

SE in slang uses

In exclamations

stiffen the lizards! (also stiffen me! ...the bees! ...the cow ...the crows! ...the rooks! ...the snakes! ...the wombats!)

[1910s+] (orig. Aus.) an excl. of surprise, shock etc.