means A box or pit filled with sand for children to play in — or, by extension, a contained space where you can experiment safely without affecting anything important.

from A plain compound of "sand" and "box," describing exactly what it is: a box of sand. "Sand" descends from Old English "sand," with cousins across the Germanic languages. The literal children's-play sense is the older one; the figurative "safe place to test things" meaning is a 20th-century extension, picked up enthusiastically by computing, where a "sandbox" walls off untrusted code so it can make a mess in private without wrecking the rest of the system — a digital pit where your programs can dig and squabble like toddlers.