A father-in-law is the father of the person you married. Meet your father-in-law.
The plural puts the s on father rather than on law, which catches people out. The relationship survives a divorce socially and ends legally at once.
1nounfather-in-lawThe father of the person you married.
“His father-in-law taught him to fish and never mentioned the wedding.”
father-in-law is a relative.
Middle English fader in lawe; the law is church law about marriage rather than any statute.
Middle English
fader in lawe
English, today
father-in-law
From Middle English fader in lawe, fadyr-in-lay, fader yn lawe, equivalent to father + in-law.
Etymology from Wiktionary, CC BY-SA