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.
1) joining, joint (such as elbow etc) Part of Speech: noun feminine A Related Word by BDB/Strong’s Number: from H680 (in its primary sense of uniting) Same Word by TWOT Number: 153c
Brown-Driver-Briggs
[אַצִּילnoun [feminine]joining, joint (compare elbow) — plural construct [י]אַצִּילֵי ידEzekiel 13:18 (on יד compare Sm Co); — joint of hand, i.e. elbowEzekiel 13:18; אַצִּילוֺת יָדֶיךָJeremiah 38:12arm-joints. — אִַצּילָה׃Ezekiel 41:8 obscure; perhaps ׳א is here a technical architectural term to the joining (compare Sm Ke).