From De Lingua Latina, 6. 69-70
“Spondere is to say spondeo,”I promise”, related to sponte, something done willingly—this has the same force as a voluntate, “with personal inclination”. This is why Lucilius writes about the woman from Crete that she came to his bedroom willingly, that she tossed off her clothes of her own desire. Terence intends the same willingness when he says that it is better: “to do something right because of your own correct desire rather than fear of another.”
From the same sponte on which spondere is based, are derived the words despondet (“he pledges”) and respondet (“he promises in return, answers”), desponsor (“promiser”), and sponsa (“promised bride”), and many others that are similar. For one spondet (“solemnly swears”) when he says sponte (“willingly”) spondeo (“I pledge”). He who has promised (spondidit) is thus a sponsor. He who is by “formal promise” (sponsus) bound to keep a pledge to another person is called a cosponsus.
This is what Naevius is thinking when he says consponsi. If money or a daughter “were promised” (spondebatur) as part of a marriage arrangement, both the money and the girl who was promised (desponsa) would be called sponsa (“pledged”). The money which had been agreed upon under the engagement agreement (sponsu) was called a sponsio (“guarantee”); the man to whom the things were promised would be called a sponsus (“betrothed”) and the day of the agreement would be called “betrothal day” (sponsalis).
Spondere est dicere spondeo, a sponte: nam id idem valet et a voluntate. Itaque Lucilius scribit de Cretaea, cum ad se cubitum venerit sua voluntate, sponte ipsam suapte adductam, ut tunicam et cetera reiceret. Eandem uoluntatem Terentius significat, cum ait satius esse
Sua sponte recte facere quam alieno metu.
Ab eadem sponte, a qua dictum spondere, declinatum despondet et respondet et desponsor et sponsa, item sic alia. Spondet enim qui dicit a sua sponte “spondeo”; qui spopondit, est sponsor; qui idem ut faciat obligatur sponsu, consponsus.
Hoc Naevius significat cum ait “consponsi.” Si spondebatur pecunia aut filia nuptiarum causa, appellabatur et pecunia et quae desponsa erat sponsa; quae pecunia inter se contra sponsu rogata erat, dicta sponsio; cui desponsa quae erat, sponsus; quo die sponsum erat, sponsalis.