My husband went into our backyard in just his underwear yesterday. Does this mean he is senile?

If this is the only sign, probably not. He may have just been distracted. Watch for real personality changes and significant, repeated memory loss, before you get too concerned. Changes in behavior are significant in Alzheimer's disease, but one incident doesn't mean a whole lot.
Take care,
Carol

Maybe. Maybe it was just a senior moment. You can't base much on a single action. Just keep alert and see if there is a continuing pattern of "untypical" behavior.

I would Recommend Trading his Panties for boxer underwear/shorts. they are acceptable everywhere. BTW, i check the mai in my nightgown and go outside in my nightgown. Its my yard and I aint naked. BUT yes, boxers are a little more coverage than Whitey Tighties.

I'd ask him why he did it. I'd ask him if this is a new thing that the two of you are doing in the future. If he asks you what the heck you're talking about, then I'd say you're right, he's probably showing signs of dementia.

How is he other than that, does he forget, get angry , or talk without reasoning? My Mom went thru that a bit, it didnt bother her to walk around with no pants on, so unlike her, she was so modest! I ended up buying unstrippable clothing for that phase. Good luck, just watch and observe for now.

I would have to agree with the others, if there isn't any other different behaviors, I would watch and see.

No.

My recommendation would be to watch and monitor his behavior. Could have just been a senior moment or it could of been early stages of dementia. What did he do when he realized he was in the backyard in his underwear? For that matter did he even realize he had made a mistake? Wouldn't worry to much at this stage. However, you might want to discuss this with his doctor. There are medications that might help and slow the process if it is dementia.