Lewis & Short

sŭbĭtārĭus, a, um, adj. [subitus], done suddenly or in haste, sudden, hasty: ad eum exercitum explendum Latini Hernicique dare Quintio subitarios milites (ita tum repentina auxilia appellabant) jussi, Liv. 3, 4, 11; so, milites (corresp. to tumultuarii), id. 40, 26, 6: exercitus, id. 3, 30, 3; 31, 2, 6; 41, 17, 9: res, Plaut. Mil. 2, 2, 70: aedificia, Tac. A. 15, 39: gradus theatri (with scena in tempus structa), id. ib. 14, 20: dictio, a speaking extempore, Gell. 9, 15, 5: consilium, App. M. 9, p. 228, 17.
Plur. subst. n.: subitaria ista nostra, off-hand sayings, App. de Deo Socr. p. 104.