The name Overbourn is not explained, though it at least seems clear that -bourn (Old English burna, 'stream') is a reference to the river Shirebourn on the marshes' northern edge. On the simplest level, the name might suggest that the marshes were 'over the (Shire)bourn)', as seen from within the lands of the Shire.

In Old English, ōfer carried various meanings, and perhaps the most applicable here would be 'along', so we could read ōfer burna as 'along the stream'. Alternatively, ōfer could mean 'border' or 'bank', giving a meaning of 'border stream' or 'river bank'. Any of these alternatives would seem to be at least plausible.

One final possibility is that the name represents a playful pun of the kind popular among the Shire-hobbits. On this reading, the name Overbourne might be meant to bring to mind the word 'overborne' (in the sense of 'overwhelmed' or 'overpowered'). In that sense, the waters of Brandywine were 'overborne' by those of the Shirebourn, and so flooded out to form the marshes. This derivation seems a little more far-fetched than the Old English alternatives, but such punning names were far from unknown among the Hobbits.