Buff is a cool word because its etymology comes from a single source, jumps all over the place, and is pretty easy to follow.

1. noun -- Originally a type of leather made from buffalo skin. From the French word buffle, meaning buffalo*. Later buff was used to refer to other types of leather, particularly leather made from ox hide and softened with oil.

2. adjective -- A light yellow brown or tan color. From the color of buff leather.

3. noun -- A fan, collector, or enthusiast (as in "He's a real music buff") From the buff colored coats of the New York City firefighters. From the 1820s into the early 1900s the volunteer firefighters wore these coats, earning the name 'fire buffs'. Eventually buff was generalized so that you can now refer to people as music buffs, sports buffs, computer buffs, opera buffs, etc., etc.

"The Buffs are men and boys whose love of fires, fire-fighting and firemen is a predominant characteristic."
N.Y. Sun, Feb. 4, 1903

4. noun -- Bare skin. 'In the buff' means naked. From the association between skin (buffalo hide) and nudity, plus the fact that buff leather is about the same color as European standard skin tone. This meaning dates back at least 400 years.

"I go in stag, in buff"
Thomas Dekker, 1602.

5. verb -- To buff is to polish. From the practice of polishing metal with leather in order to make it shine. We still buff metal (although not often with leather), but these days we are just as likely to buff our shoes.

6. noun, archaic -- Like the New York firemen, there was also a regiment in the British Army that were popularly know as the Buffs due to the color of their uniforms (and later, the facings on their uniforms). In 1744 they were officially named 3rd Regiment of Foot, but were better known as Howard's Buffs. By the mid-1700s, this had been shorted to simply 'The Buffs". there were a number of variations on this name throughout the years (the Old Buffs, the Young Buffs, the Queen's Own Buffs), but for those of us who are not military buffs, we need only note that this is the origin of the phrase "Steady The Buffs!". We don't know exactly where this phrase came from, but we do know that Kipling used it in Soldiers Three (1888). "Steady the Buffs" was a common phrase to express warning or encouragement in the early 1900s.

7. adjective -- I've had a lot of people /msg me asking about buff in the sense of physically fit, well muscled, or physically attractive. I hadn't found anything on the etymology of this sense of the word buff, but all the requests prodded me into searching a bit more, and voilĂ ! This sense of buff apparently comes from the world of weight training, and was originally not 'buff', but 'buffed'. While not conclusive, this suggests that it derives from sense 5. above.

* This is not the American bison, but the Indian water buffalo. The word buff made its way into the English language in this form before the word buffalo (1550 and 1588, respectively). While we got buff from the French buffle, we probably got buffalo from the Portuguese bufalo. These both come originally from the Latin bufalus.