buckaroo n.

"male deer," c. 1300, earlier "male goat;" from Old English bucca "male goat," from Proto-Germanic *bukkon (source also of Old Saxon buck, Middle Dutch boc, Dutch bok, Old High German boc, German Bock, Old Norse bokkr), perhaps from a PIE root *bhugo (source also of Avestan buza "buck, goat," Armenian buc "lamb"), but some speculate that it is from a lost pre-Germanic language. Barnhart says Old English buc "male deer," listed in some sources, is a "ghost word or scribal error." The Germanic word (in the sense "he-goat") was borrowed in French as bouc.

The meaning "a man" is from c. 1300 (Old Norse bokki also was used in this sense). Especially "fashionable man" (1725); also used of a male Native American (c. 1800) or Negro (1835). This also is perhaps the sense in army slang buck private "private of the lowest class" (1870s).

The phrase pass the buck is recorded in the literal sense 1865, American English poker slang; the buck in question being originally perhaps a buckhorn-handled knife:

The 'buck' is any inanimate object, usually [a] knife or pencil, which is thrown into a jack pot and temporarily taken by the winner of the pot. Whenever the deal reaches the holder of the 'buck', a new jack pot must be made. [J.W. Keller, "Draw Poker," 1887]

The figurative sense of "shift responsibility" is first recorded 1912; the phrase the buck stops here (1952) is associated with U.S. President Harry Truman.

"factitious slang suffix" (OED), sometimes affectionate, forming nouns indicating "a humorous or remarkable instance" of what is indicated, in use by 1940s (e.g. smasheroo "spectacular success," by 1948), perhaps from buckaroo. An earlier suffix in a similar sense is -erino (after 1900), apparently from -er + Italian suffix -ino.