口吸い (hiragana くちすい, romaji kuchisui, historical hiragana くちすひ)
/kutisupi/ > /kutisuɸi/ > /kutisuwi/ > /kutisui/. Literally "mouth sucking". Compound of 口 (kuchi, “mouth”) + 吸い (sui, “sucking”), the nominalized form of the verb 吸う (suu, “to suck”).