One of the mysteries of the English language finally explained.
Definition of wife-swap in English:
wife-swap
noun
1A permanent or long-term exchange of wives between married couples, especially for the purpose of remarriage.
2A temporary exchange of wives or sexual partners between couples for the purpose of engaging in sexual activity.
verb
no object To exchange wives or sexual partners for the purpose of engaging in sexual activity; to engage in wife-swapping.
Origin
Mid 19th century; earliest use found in Sullivan (Indiana) Democrat. From wife + swap<br>1960s; earliest use found in Logansport (Indiana) Pharos-Tribune. From wife + swap, after wife-swap, wife-swapping.