'Troon' is just short for Duntroon....an Aussie Military 'boot-camp academy'...and we are lazy so anything/everything gets abbreviated.
A Wombat is a small [ish] burrowing marsupial which is quite 'solid' and has been known to be the cause of 'road-kill'....namely it walks away, but the car driver is the one who is killed.
The dictionary description has led to the 'joke' as a misplaced, extraneous comma can totally change the meaning....
Wombat. n. Small burrowing mammal, eats roots and leaves...