one’s arm or arms.
The stative form of this word antāka means “having arm muscles, muscular”. So to say someone is arm-y (armed? English does not have an adjective with this meaning that I can tell.), la sāen matāka, means that person is muscular.
[Note: I’m not planning on having entries for every single part of the body I have words for. I figure I will stick with the more common and more interesting ones. If you think I have missed something and want to know the word for it, let me know and I will make an entry for you.]