Someone who is acting like an asshole, or an idiot.
"John wouldn't give me a slice of pizza, he was bein a real clumpdick."
Also can be used as:
"John's clumpin it up."
Adj: (1) Someone who's acting like a real asshole.
(2)Someone who habitually says stupid comments, or does something idiotic.
"Steve won't give me any of his beer, and he's got a 30. What a clumpdick."
"Kevin tapped the keg wrong, and now it's all foam."
"God, Kevin's such a clumpdick."