could not train a choko vine to grow up a dunny wall

Australian Slang

catch-phrase denoting a person's stupidity and uselessness (choko vine was once ubiquitous to Australian backyards, they were especially popular during the depression as they took little if any looking after (except perhaps for cutting back) and thus provided a cheap source of food; a tenacious and quick-growing plant it commonly spread from its source over and along fences, trellises, sheds, carports, and, naturally, outside dunnies)