Possibly from a phrase such as *reque proque (“back and forth, to and fro”), from re- (“back”), prō (“forwards”) and -que (“and”).
reciprocus (feminine reciproca, neuter reciprocum); first/second declension
- back and forth
- reciprocus in Charlton T. Lewis and Charles Short (1879) A Latin Dictionary, Oxford: Clarendon Press
- reciprocus in Charlton T. Lewis (1891) An Elementary Latin Dictionary, New York: Harper & Brothers
- reciprocus in Gaffiot, Félix (1934) Dictionnaire Illustré Latin-Français [Illustrated Latin-French Dictionary], Hachette
- De Vaan, Michiel (2008) Etymological Dictionary of Latin and the other Italic Languages (Leiden Indo-European Etymological Dictionary Series; 7), Leiden, Boston: Brill, page 516