Maybe for LARP, but the stuff I have (made in the style of Medieval armors from the 14th as well as from the 15th century), you would have been wearing linen brais (underwear), chausses (think thigh-high stockings) or hosen (joined chausses) of, usually, linen or wool. Depending on timeframe, you might have maille going all the way to your knees (mostly pre-15th), or might have either maille brais or a maille fauld (skirt) if you were wearing full plate (15th/16th). Regardless, there was no way you were getting out of it quickly. You'd most likely either crap yourself, or do your best to hold it, while armored for combat. Never seen a repro or reconstruction with anything like the backflap you see in longjohns, but with the way layers overlap and get affixed, it would still be a pain to get to them. Asian armor and under armor might be constructed differently, but I'm more familiar with European.
If you didn't have your armor on, or were merely wearing something like a jack/jupon/aketon/gambeson (textile padded items which might be worn under maille, plate or by themselves), you were more likely to be able to get them hiked up like a skirt and get your brais down to do your business. "Leather armor", such as in the picture, has no historical basis in surviving inventory records, although "buff coats" - very thick and stiff leather coats worn by 17th century cavalry - would turn a blade and, while not entirely "bullet proof", could help keep you from getting killed by the muskets of the day. They'd likely not penetrate at longer range, and might slow things down enough to keep you from getting badly wounded at closer range.