Once written quite explicitly as a pictograph of a slave (a person with clearly drawn testicles and a tattooist's needle) carrying a container with bits in it. Some authoritative Japanese scholars feel this container-with-bits radical is a chamber-pot with turds (really!). It is from this humble original meaning, filtered down through Japanese culture, that boku eventually began to me, "I."
Now it is simplified as a man showing hands holding a basket.
As used in kooboku (public servant), doboku (manservant), bokura (we).