protinus

Protinus is an adverb built from pro plus tenus, "forward as far as", and the spatial sense is the original: straight ahead, without a break in the line. The temporal sense, "immediately", is the same idea applied to time, with no gap between one event and the next. The spelling protenus turns up in older manuscripts and means the same thing.

Against its rivals: statim is the neutral "at once", subito and repente carry surprise, and protinus stresses that nothing came in between. It pairs readily with ut or ubi to mean "as soon as" (protinus ut vidit). Verse writers like it because it fills the front of a hexameter neatly, and Virgil uses it constantly to keep a narrative moving without a connective.