http://en.wikipedia.org/wiki/Erogenous#ArmsThe skin of the arms, and specifically the softer skin of the inner arms and across the creased mid-arm bend covering the ventral side of the elbow, are highly sensitive to manual or lingual stimulation. Light stroking with fingers or tongue, more vigorous kneading, and butterfly kissing can initiate arousal and, in some cases, induce clitoral/vaginal orgasm or penile ejaculation without direct contact with the latter areas. The mid-arm bend is especially sensitive due to the thinner skin found there, which makes nerve endings more accessible. Arm sensitivity may be reduced or concentrated to a more narrow range by excessive muscularity or obesity on the one hand, or transformed to uncomfortable tenderness by excessive thinness on the other.
(emphasis added) WTF, i never knew that.