The meaning of "expired" in this sentence is "past the date of being safe to drink."
The sentence shows that Benjamin made an awful face when he sniffed the milk, indicating that the milk was not in a good condition. Additionally, Benjamin exclaimed, "This is definitely expired!" This suggests that the milk in the jug has gone beyond the date when it is safe to consume. Therefore, the meaning of "expired" as used in this sentence is "past the date of being safe to drink."