Silly answer: Something that adults do
表面上 adultery 似乎與 adult(成人)相關。
Actually, adultery is related to adulterate: to add something to food or drink to make it less pure. Adultery is similar: adding an outside person to a marriage, this making it less pure, a violation of the 7th Commandment.
事實上 adultery 和 adulterate(摻雜使品質變劣)有點類似:加入第三者使婚姻變得脆弱。