The expression Kaiawase is not used against lesbians but is used in street jargon to refer to lesbian intercourse derogatorily. We can translate it into the English language as bumping pussies because the word Kai, meaning the shell, is also vulgar slang for vagina. Besides, there is a traditional Japanese game with the same name, in which you must combine the shells one against each other. So, the word Kaiawase could also have the meaning of meeting pussies. Another related expression, even more potent, is the word Ainame, which could make the meaning of mutual licking.