What it means
A humblebrag is when someone sneaks a brag in through the back door by dressing it up as a complaint or fake modesty. They act like they're moaning, but really they're polishing their own halo and hoping everyone clocks the flex.
Usage examples
"Ugh, I hate how my new flat gets too much sunlight, total humblebrag."
"Saying you are exhausted from being so popular is a humblebrag, not a problem."
"She posted, Can't believe I have to choose between two job offers, absolute humblebrag."
"Mate, saying your designer shoes are such a pain to keep clean is not a struggle, it's a humblebrag."
Where it comes from
It’s a straight blend of humble and brag. The word was around earlier, but it really took off in the late 2000s and early 2010s thanks to comedian Harris Wittels, who popularised it by collecting and mocking these fake-modest flexes online and in interviews.