When it comes to synonyms for the phrase "undress up to the underwear," there are a few options to choose from. One option could be to say "strip down to the skivvies," which implies a more casual or playful tone. Another option could be to use the more straightforward phrase "remove clothing until only underwear remains." Additionally, one could say "peel off layers until only underwear is left," or "shed garments to reveal only undergarments." Whatever phrasing is used, the meaning remains the same: removing clothing until only underwear is left.