Haven’t heard of it? Harris Wittels explains:
A Humblebrag is basically a specific type of bragging which masks the brag in a faux-humble guise. The false humility allows the offender to boast their “achievements” without any sense of shame or guilt. Unfortunately/fortunately, Humblebragging is very commonly used in our society, and for some reason Twitter seems to be the perfect forum for people to do it.