dossum

Colloquial form. For the /rs/ > /ss/, cf. prōsum, rūsum, sūsum (with subsequent shortening of /ss/ after long vowels) and the Romance descendants of deorsum or ursus.

dossum n (genitive dossī); second declension (proscribed)

1. Alternative form of dorsum (“back”)
   • [98–138 CE, Velius Longus, edited by Henricus Keilius, Liber de orthographia (Grammatici Latini; VII)‎, published 1880, page 79, lines 4–6:

     sic et dossum per duo s quam per r dorsum quidam ut lenius enuntiaverunt. ac tota r littera sublata est in eo quod est rusum et retrosum.

     And in this way, some [say] "dossum" with two s's instead of "dorsum" with an r to sound gentler. But to all, r is a sunk letter in "rusum" and "retrosum".]

Second-declension noun (neuter).