Used casually in conversation to tease someone about their poor skills. You missed the goal again? You're quite the duffer, aren't you?
Pro Tip 2/3
Avoid Offense
Be careful using 'duffer' as it might offend someone who is sensitive about their abilities. I hope you don't mind the joke; I didn't mean to call you a duffer seriously.
Pro Tip 3/3
Humorous Undertone
Often has a playful or humorous tone rather than being harshly critical. My grandpa calls himself a duffer when he misses a putt.