A person who goes through their life very similarly to how a bumblebee flies- slow, clumsy, and troubled. Aerodynamically impossible, but somehow they manage.
They can be well-meaning jackasses who make stupid mistakes, or complete idiots who have no idea what they're doing, but continue to pursue their dreams of pollination. Sometimes they're endearing- it takes all types.
You don't want them near delicate objects, or within 50 feet of a china shop.
Bill, that bumblefuck, ended up burning the eyebrows off his face with a blowtorch last Tuesday in effort to make an ice sculpture for a girl he loved.
Nancy, head bumblefuck of the local high school district, invested a large percentage of the school's money in stocks that failed.