A more thorough explanation:

A cohabiting unmarried person of the opposite sex is someone who lives with a partner of the opposite sex without being married. This term is often abbreviated as CUPOS.

For example, if John and Jane live together but are not married, they would be considered CUPOS.

This term is similar to POSSLQ, which stands for "person of the opposite sex sharing living quarters." However, CUPOS is more precise because it specifically excludes married individuals.

Overall, CUPOS refers to a couple who lives together and shares a household but is not legally married.