term of address, originally (1883) nautical and with a sense of "swaggering, domineering fellow." Probably from buck (n.1) in the slang sense of "a blood or choice spirit."
There are in London divers lodges or societies of Bucks, formed in imitation of the Free Masons: one was held at the Rose, in Monkwell-street, about the year 1705. The president is styled the Grand Buck. ["Dictionary of the Vulgar Tongue," 1811]