Husband (32M) and his friends have a nickname for me (35F) in their group chat, he won't tell me what it means.

This happened a couple days ago. My phone died right as I was texting my mom about something rather important so my husband let me use his. As I was typing, a banner popped up with a text from one of his friends to their group chat, saying to the effect of "hey (Husband), do you and ST still want to (halloween plan we'd talked about)?"

So of course I'm like who's ST, those are not my initials. I briefly looked at the chat. A few messages up I was at least able to confirm "ST" was me, in context of my husband mentioning a specific thing I'd done that day.

Like I said S.T. is not an abbreviation of my name or any name I would go by. None of our friends calls me by anything other than my real name. So I was confused. I showed him the chat and asked what it stood for.

He looked suddenly really flustered. He grabbed his phone back and said "oh uhh it's nothing, just a nickname." Okay? Well what's it stand for? He literally wouldn't tell me...he just kept saying "don't worry about it." Well is there some reason I would need to worry? I wouldn't be worrying if he just explained what it meant... I was a little put off by this, to find out all his friends are calling me something I don't even know about? I told him even if it's just some dumb joke at least tell me. Because clamming up just makes it look kind of weird. He still refused to tell me. This entire back and forth was maybe a minute or two. Then, he suddenly goes, "Okay fine it's Super Terrific."

Umm..... Look, I can't tell if I'm being crazy but I just don't fully buy that? If that's all it was, why not say so the first time I asked? I asked like 3 or 4 times for him to tell me what it stood for. Why keep avoiding it? On the other hand I know this makes me look like a really high strung paranoid person to be suspicious when it could so easily be what he said. I don't want to accuse him of lying with no proof, and over something so stupid.

This is still bugging me a little. I just want to know if I'm being insane to have an inkling of doubt about it standing for Super Terrific. Do you think he was telling the truth? Would it be ridiculous to bring it up again at this point?

TL;DR: My husband's group chat refers to me as "S.T." which my husband says is for Super Terrific, I don't fully believe he's telling the truth. I'm worried I'm being dramatic to keep overanalyzing his reaction when I asked and thinking about what else it could mean.

Edit: I'm not sure what's going on, but none of my comments are showing up. I logged out and saw that none of them appear even though you can see them on my profile. So, if you've asked a question or given suggestion please check my profile for a response. I'll try to keep making replies and hope they eventually show up.