What does it mean when someone calls you a sponge target?

The other day some of my son's friends were over and I heard them arguing, so I went to check it out I didn't interfere i wanted to see how he handled it. But I guess he was angry because they said I was there sponge target? After I took them home I tried talking to him about it but he didn't want to talk. He's 13 and me and his father just recently filed for divorce so I didn't want to push it. I couldn't find anything on the internet about a sponge target tho any thoughts?
What does it mean when someone calls you a sponge target?
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