Lewis & Short

rōrārĭi, ōrum, m. [ros] (sc. milites), a kind of light-armed Roman troops, who usually made the first attack and then retired, skirmishers (cf.: velites, ferentarii): rorarii dicti ab rore, qui bellum committebant ante, ideo quod ante rorat quam pluit, Varr. L. L. 7, § 58 Müll.; cf. Fest. s. h. v. p. 264 ib.; Non. 552 fin.; Liv. 8, 8, 8; 8, 9, 14; Plaut. Fragm. ap. Varr. 1. 1.; cf. Becker, Antiq. III. 2, p. 275.
In sing., Lucil. ap. Non. 553, 2 sq.
Hence, ‡ rōrārĭ-us, a, um, adj., of or for the rorarii: rorarium vinum, quod rorariis dabatur, Fest. p. 267 Müll.