Excursus comes from Latin excursus ('a running out, excursion, digression'), from excurrere ('to run out'), from ex- ('out') + currere ('to run'). Morphologically, the form is compositional: ex- marks part of the visible structure, while the remaining element supplies the lexical base. In present use, it is used as a noun meaning "a detailed digression appended to or inserted within a text", so the historical material remains connected with the current sense rather than standing as bare ancestry. The wider English family includes related forms such as excursion, excursive, and course, showing how the same historical material continues to organize neighboring vocabulary. The named historical forms include Latin excursus, Latin currere, and Proto-Indo-European *ḱers-, keeping the history tied to concrete forms rather than a loose resemblance. Its semantic neighborhood includes digression, aside, disquisition, and appendix, locating the modern specialization without adding a new origin story.