A son-in-law is the husband of your daughter. Get on with your son-in-law.
The plural is sons-in-law, which is the part everybody gets wrong. A son-in-law was historically expected to take on obligations as well as a wife.
1nounson-in-lawThe husband of your daughter.
“The son-in-law did all the driving for twenty years.”
son-in-law is a relative.
Son plus in-law, meaning related through canon law rather than by birth.
Middle English
sone in lawe
English, today
son-in-law
From Middle English sone in lawe; equivalent to son + in-law.
Etymology from Wiktionary, CC BY-SA