The only free and comprehensive online etymological dictionary of the Spanish language
cárcel m. (Noun) "prison"
10th cent. Old Spanish cárcere 'id.' From Latin carcerem, accusative of carcer 'id.' From Proto-Italic *karkro- "enclosure." From Proto-Indo-European *kr̥-kr- 'id.,' a reduplication of a root *ker- "circle" that otherwise only attested in Greek. *ker- may have been a loanword.
Asturian cárcel, (dialects) cárcere Portuguese cárcere, Galician cárcere, Italian carcere, Romanian carceră
Ancient Greek καρκίνος (karkínos) "crab"