mold consisting of a box with sand shaped to mold metal
"The artisan carefully packed the wet mixture into the wooden sandbox before pouring in the molten copper to cast the new gear."
A children's play area consisting of a box filled with sand.
"The little girl spent her afternoon building castles in the sandbox while her parents watched from the patio."
To restrict (a program, etc.) by placing it in a sandbox.
"The security team decided to sandbox the new software update before deploying it to all servers to prevent any potential malware from spreading."
The word "sandbox" comes from combining the words "sand" and "box." It originally referred to a literal container filled with sand.