Lewis & Short

prīmŭlus, a, um, adj. dim. [primus], the first (ante-class.): primulo diluculo, Plaut. Am. 2, 2, 105.
Adv.: prīmŭlum, at first, first (ante-class.), Plaut. Men. 5, 5, 18; 5, 9, 57; Ter. Ad. 3, 1, 2.