swéḱuros
“father-in-law”father-in-law
PIE word for father-in-law. Gives Latin socer, Greek hekurós, Sanskrit śváśura, English (archaic) "sweher".
Discussion
The Proto-Indo-European word *swéḱuros meant "father-in-law" and belongs to the remarkably well-preserved system of affinal kinship terminology — words for relatives by marriage — that is one of the most impressive achievements of Indo-European comparative linguistics. The reconstruction of affinal terms tells us that the Proto-Indo-European speakers had a structured system of marriage alliances with specific vocabulary for in-law relationships.
In Germanic, the initial *sw- was preserved and the palatal *ḱ developed into a velar. Old English swēor "father-in-law" is the direct reflex, though this word fell out of use in English during the medieval period and was replaced by the compound father-in-law. German Schwäher "father-in-law" (now archaic) preserves the cognate. Some scholars have connected the word swear to this root family (through a chain involving sacred oaths and paternal authority), but this etymology is highly contested and most authorities derive swear from a different source.
Latin socer "father-in-law" is a clear cognate, as is Greek hekuros (with regular loss of initial s-). Sanskrit śváśura- "father-in-law" confirms the Indo-Iranian form, and the word is attested in Old Irish (sethir), Lithuanian (šešuras), Old Church Slavonic (svekrŭ), and Armenian (skesur), making this one of the best-attested kinship terms across the entire Indo-European family.
The existence of specific Proto-Indo-European terms for father-in-law (and mother-in-law, *sweḱrúh₂) provides important evidence about Proto-Indo-European social structure. Most scholars infer a patrilocal marriage system, in which the bride moved to the husband's household. The bride would need specific terms for her new husband's parents — hence the development of dedicated in-law vocabulary. The fact that these terms are so well preserved across branches suggests that the marriage system they encoded was both ancient and fundamental to Proto-Indo-European society.
Notes
Part of the detailed PIE in-law terminology reflecting patrilocal marriage