Dear Pure Heart and Mind --
I think that would depend on how you wish to apply discipline. If you wish to control someone, then in most cases, i would say that you would apply it to your hands. If you wish to metaphorically or actually step on someone, apply it to your feet. It could also be used to dress any article or object of mastery or discipline.
In addition, remember that the word "master" has another meaning -- to thoroughly learn a topic or be able to accomplish a feat, as in mastering long division or becoming a master of martial arts. So in these cases, Master Oil could be applied to whatever body part used in training for and attaining such accomplishments.
OIL-HOO-MAST
Master Oil
$7.50
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Good luck.