"travelling south," 1872, originally in railroading, from south + bound (adj.2).
Entries linking to southbound
south adv.
Old English suþ "southward, to or toward the south, southern, in the south," from Proto-Germanic *sunthaz, perhaps literally "sun-side" (source also of Old Saxon, Old Frisian suth "southward, in the south," Middle Dutch suut, Dutch zuid, German Süden), and related to base of *sunnon "sun" (from PIE root *sawel- "the sun"). Old French sur, sud (French sud), Spanish sur, sud are loan-words from Germanic, perhaps from Old Norse suðr.
As an adjective, "being or situated in the south," from c. 1300. As a noun, "that one of the four cardinal points directly opposite to north," also "southern region of a country," both late 13c.
The Southern states of the U.S. have been collectively called The South since 1779 (in early use this often referred only to Georgia and South Carolina). South country (late 14c.) in Britain is below the Tweed, in England below the Wash, in Scotland below the Forth. The South Sea meant "the Mediterranean" (late 14c.) and "the English Channel" (early 15c.) before it came to mean (in plural) "the South Pacific Ocean" (1520s) and it was frequent for "the Pacific Ocean" generally in U.S. in early 19c. (Thoreau, J.Q. Adams, etc.).
bound adj.2
c. 1200, boun, "ready to go;" hence "going or intending to go" (c. 1400), from Old Norse buinn past participle of bua "to prepare," also "to dwell, to live," from Proto-Germanic *bowan (source also of Old High German buan "to dwell," Old Danish both "dwelling, stall"), from PIE root *bheue- "to be, exist, grow." The final -d is presumably through association with bound (adj.1).