Sam was responding to fans who urged Britney to get a prenup ... his response -- yep, they'll sign an "iron clad prenup," adding, "to protect my jeep and shoe collection in case she dumps me one day."

Now, Sam added a few crying laughing emojis, but it's actually a real issue.

As we reported, she broke the news on Instagram, posting a video of her showing off the huge rock on her finger, with the caption ... "I can't f***ing believe it 💍💍💍💍💍💍❣️!!!!!!"

It's worth noting ... Britney signed a prenup with Kevin Federline, so it's something she's clearly inclined to do ... at least in the past. Although these stories on net worth are often wildly inaccurate, Brit's reported estimate worth hovers at around $60 million. So, Sam's clearly joking about a prenup that protects him, but that doesn't mean a prenup is not in their future.

BTW ... since the judge in Brit's conservatorship case already said she's capable of hiring a lawyer, it seems the coast is clear for her to hire another lawyer to handle prenup business. Here's the wild card -- the conservator of her estate -- as long as the conservatorship exists -- could demand a prenup be signed to protect her assets, and right now that person is Jamie Spears.