A duffer is a lovable blunderer, someone who’s rubbish at a particular job and still gives it a go. It’s a mild, old-school jab, the kind you’d hear on a golf course, in the shed, or when someone’s DIY has gone spectacularly wrong. Can also mean a poor player, especially at golf.
"Swore I’d fix the leaky tap. Ten minutes later the kitchen’s flooding and Dad just sighs, you duffer, hand us the spanner."