Side-wife

If, however, his wife gave him a handmaiden and he had children by her, he was then barred from taking a side-wife (as a concubine was called).

Source: wiktionary

She was also a 'sidewife'. Nobody talks about it because she is a rich entrepreneur.

Source: wiktionary

A gilly (or sheevin) is a side-wife taken by a man who already has a legal wife.

Source: wiktionary

For one who has no husband, as well as for one who acts as a side-wife for a person, who has formed a household, or who lives elsewhere and is not enumerated together [with a husband or children on a population register], […]

Source: wiktionary