large field; broad open field
A literal noun meaning a large field or broad open plain. As an everyday common noun it is uncommon; modern Japanese will often use a phrase such as 広い野原 instead.
In older writing, 大野 can refer to a broad field or plain.
野 is the plain, general word for a field or open country, whereas 大野 specifically emphasizes its large size.
Mya arenaria oonogai; soft-shell clam subspecies
The spelling transparently combines 大, “large,” with 野, “field” or “plain.” No historical etymology beyond this component meaning is specified.