1. tressis — de Vaan
tressis 'the sum of three asses' (Varro+), ndnussis 'the sum of nine asses' (Varro+). The nom.sg. probably was *ass. Originally a rectangular bronze plaque weighing a pound. In imperial times, it became a round coin. EM suspect a loanword, maybe from Etruscan, because libra and nummus would also be loanwords. For bes, WH assume a compound of *duo + gen.sg. *ass(is) 'two parts of an as' > *duass > *duess > *dwess > … — [de Vaan, s.v. tressis, p. 71]