hi! jjukkyu is just the union of two of the syllables of their names pronounced adorably: jju 쭈 (from jun 준) and kkyu 뀨 (from gyu 규). jjukkyumiz 쭈뀨미즈, if i'm not mistaken, is the friendship name they gave themselves during a joint live performance, when a m0a asked what they should call their airpods, and they answered jju (yeonjun) + kkyu (beomgyu) + m0a (me, transliterated as mi), as in the three of them being together. the z sound at the end, i'm sure you know, is similar to the one you can find in other friendship names like danjjakz/bestiez.
i'm no expert!! but i'm pretty sure both terms, and specially the second (is the one i've seen the most, tbf), are used in sk strictly as the friendship pairing, while yeonbam/yeongyu and bamjun reference the ship pairing in their different roles as top/bottom, respectively.
hope this helps!