Root/Stem: *swek'ur- / *swek'ru'-
Meanings:  husband's father / mother
Cognates (18):
Hellenic Greek hekuros (husband's father)
Italic Latin socer (husband's father), socrus (husband's mother)
Celtic Welsh chwegr (husband's mother), Cornish hweger
Indic Sanskrit çvaçura- (husband's father)
Dardic & Nuristani
Iranian Avestan xvasura- (husband's father)
Armenian Armenian skesur (husband's mother), skesrair (husband's father)
Balkan Albanian vjehe"r, vjer (husband's father)
Germanic Common Germanic *swixaraz, >
Gothic swaihra (husband's father), swaihro (husband's mother), Old High German swehur (husband's father), Old English swéor (husband's father), sweger (husband's mother), Old Icelandic swæra (husband's mother);
German Schwiegervater, Schwiegermutter
Baltic Lithuanian šešuras (husband's father)
Slavic Common Slavic *svekürü (husband's father), *svekry (husband's mother); >
Russian svekor (husband's father), svekrov' (husband's mother)
Notes: The Proto-Indo-European language, like a lot of ancient languages, had different words for dozens of family relatives. Later, when languages of the fasmily started turning into analytic ones, those relative terms changed into neutral and abstract words like 'father-in-law', 'brother-in-law', 'grandfather' etc. 
Archaic Indo-European languages still preserve distinction between the words 'husband's father' and 'wife's father': for example, Russian svekor and test', which have nothing in common etyomologically and derive from completely different stems.
The word is very ancient, it usually belonged to o-stems, and its feminine counterpart - to ú-stems (like in Slavic, Italic, Armenian).