Southbound is a compound of south + bound ('headed for, ready to go'), where this -bound derives from Old Norse búinn, past participle of búa ('to prepare, make ready'). The historical chain includes Old English sūþ and Old Norse búinn. Older glosses in the chain include "headed for, ready to go" and "to prepare, make ready", which help explain the semantic range of the modern word. Morphologically, southbound is best read as compound and borrowing, yielding an English adjective shaped by both form history and later usage. In current English it functions as an adjective, with the sense "traveling or directed toward the south". related forms include south, southward, and southerly. The result is an English adjective whose etymology preserves the older forms while supporting the word's specialized modern meaning.