It all went down on the YouTube series -- where guests chomp on chicken wings drenched in hotter-than-hot sauces ... and Heidi was the latest to step up to the plate, holding her own on Thursday's episode. That is, until she came across one wing that got her stripping.

As you can see, Heidi was already sniffling a bit from the spicy sauces beforehand ... but when she took a shot at the "Da'Bomb" hot sauce, everything hit the fan -- and that's when her shirt started to come off.

Heidi started freaking out, downing buttermilk like there was no tomorrow. But, when that didn't cut it, she unbuttoned her blouse and fanned it over her bra-exposed cleavage.

No doubt, it was a total first for the show, catching everyone off guard -- even host Sean Evans himself -- who confessed he was feeling the heat too .... but for different reasons.

Heidi chugged down more milk, but still, she was struggling -- but when Sean mentioned the fiery sensation would last another 5-10 minutes, she decided she couldn't tough it out for that long -- and without missing a beat, the shirt completely came off too.

Despite her desperate attempts to cool down, she was still not composed ... groaning about her mic falling in her lap and trying to push the hair out of her face with fingers covered in spicy sauce.

It might've seemed staged, but Heidi apologized for her impromptu strip-down and urged the kids watching to keep covered up too -- so looks like she wasn't ready for the fiery turn.