Lewis Short
(adj.adv.) : millēsĭmus (-lensimus), a, um, adj. mille
* The thousandth (class.): millesimam partem vix intellego,Cic. Att. 2, 4, 1; cf.: in millesimo corpore,Cels. 2, 6: inter mille rates tua sit millesima puppis, i. e. ultima, Ov H. 13, 97: usura,one for every thousand monthly,Sen. Ira, 3, 33: pagina,Juv. 7, 100. —In plur.: armillam, ex millesimis Mercurii factam,the thousandth part of gain vowed to Mercury,Petr. 67, 7.—Hence, adv.: millēsĭmum, for the thousandth time: Q pater quartum vel potius millesimum nihii sapit,Cic. Att. 12, 5, 1.
Charlton T. Lewis, Charles Short, A Latin Dictionary