For years, scientists have been studying why armpits are so vile. Along with all the bacteria and apocrine, eccrine and hair glands present under the crook of your arm, underarm sweat contains androstenone and other steroids. Androstenone is found in both males and females, and, for what it's worth, boar saliva.
In humans, androstenone produces a rather unpleasant musky, woody smell. It's also associated with the scent of urine [sources: Yamazaki et al., Araneda and Firestein]. On the flip side, some folks perceive its scent as sweet and floral, and still other sniffers can't smell it at all [source: Araneda and Firestein].