From 'atsal (in its primary sense of uniting); a joint of the hand (i.e. Knuckle); also (according to some) a party-wall (Ezek. 41:8) -- (arm) hole, great.
see HEBREW 'atsal
H679. atstsil
[אַצִּיל noun [feminine] joining, joint (compare elbow) — plural construct [י]יד אַצִּילֵי Ezekiel 13:18 (on יד compare Sm Co); — joint of hand, i.e. elbow 13:18; יָדֶיךָ אַצִּילוֺת Jeremiah 38:12 arm-joints. — אִַצּילָה׃ Ezekiel 41:8 obscure; perhaps ׳א is here a technical architectural term to the joining (compare Sm Ke).