Refers to the sometime recipient of male ejaculate resulting from a "circle jerk" (more than two men self-masturbating in a social setting, as to a porn film.) Also, it may refer to a person who masturbates two or more men simultaneously; i.e., at the centre of a circle. Usually used in a derogatory sense.
Used figuratively as a self-deprecation: "I feel like the bucket man at a circle jerk."
see above, or:
"We got a little excited watching the Jenna Jamison video, and Micah was the bucket man!"
"I don't want to be the bucket man in a Turkish prison movie!"