"beyond what is permitted by the marriage tie or vow," hence, euphemistically, "adulterous," 1833 (Carlyle), from preter- "beyond" + nuptial.
Entries linking to preternuptial
preter-
also praeter-, word-forming element meaning "beyond; over, more than in quantity or degree," from Latin praeter (adverb and preposition) "beyond, before, above, more than," properly comparative of prae "before," from PIE root *per- (1) "forward," hence "in front of, before."
nuptial adj.
"of or pertaining to marriage or the wedding ceremony," late 15c., from French nuptial, or directly from Latin nuptialis "pertaining to marriage," from nuptiae "a wedding," from nupta, fem. past participle of nubere "to marry, get married, wed, take as a husband," which is of uncertain origin. Perhaps it is from a PIE root *sneubh- "to marry, wed" (source also of Old Church Slavonic snubiti "to love, woo," Czech snoubiti "to seek in marriage," Slovak zasnubit "to betroth"). De Vaan finds the old theory that the verb nubere is literally "to cover, veil oneself" (as a bride) semantically attractive but unproven (compare Latin obnubere "to veil, cover the head," from nubes "cloud"). Related: Nuptially.
Nuptial number, a number obscurely described at the beginning of the eighth book of the "Republic" of Plato, and said to preside over the generation of men. The number meant may be 864. [Century Dictionary]