1.)The smelly brown knuckle of the finger or thumb inserted in someones butt. 2.)The person inserting said finger or thumb 3.) The act of inserting said finger or thumb in one's butt, possibly due to boredom or lack of interest. 4.)Someone who complains alot but does nothing and/or offers no suggestions about how to remedy the problem he/she is complaining about.
Dave is such a butt-knuckle that rather than do something useful he'll just sit there with his thumb up his butt complaining about everything. Hey can you help me with this right after you go wash the stink off of your butt-knuckle. Don't go butt-knuckle on us now we may still need your help before it's all over.