Firth comes from Middle English, from Old Norse fjǫrðr ('inlet, firth'), from Proto-Germanic *ferþuz, from Proto-Indo-European *pertus ('crossing, passage'), from *per- ('to cross, pass through'); a doublet of fjord. The development is chiefly a borrowing or learned continuation, carrying an older form and meaning into English rather than building the word from a purely native base. In present use, it is used as a noun meaning "a narrow inlet of the sea, especially in Scotland", 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 fjord, showing how the same historical material continues to organize neighboring vocabulary. The named historical forms include Old Norse fjǫrðr and Proto-Indo-European *per-, keeping the history tied to concrete forms rather than a loose resemblance. Its semantic neighborhood includes estuary, inlet, fjord, and arm, locating the modern specialization without adding a new origin story.