Which Latin phrase means "lightly" in the context of S.T.T.L.?

The phrase that means "lightly" in the context of S.T.T.L. is indeed "Levis." In Latin, the word "levis" translates to "light" in terms of weight or burden, often implying something that is not heavy or can be carried easily. This concept of lightness can be both literal and metaphorical, reflecting an ease or lack of seriousness in certain contexts.

On the other hand, "gravis" means "heavy" or "serious," which contrasts with the notion of something being light. "Serius" translates directly to "serious," which again does not align with the idea of lightness. Lastly, "celeriter" means "quickly" or "swiftly," relating more to speed than to the concept of lightness, thus not being applicable in this context.