noun
Mitsui Sumitomo credit card
Internet slang term formed by ateji characters 蜜 (mitsu) and 墨 (sumi), referring to a credit card issued by Mitsui Sumitomo Financial Group.
ネットで「蜜墨」って書いてあるのを見たけど、三井住友カードのことらしい。
I saw '蜜墨' written online, and it seems to refer to a Mitsui Sumitomo credit card.