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