The word underwear is commonly known as the undergarment worn for both men and women down below beneath clothes. This more modern definition means "to understand." This common noun has now become an adjective in the world of "slang" to describe comprehension.
"I do not underwear what's going on." - This sentence simply is translated to "I do not understand what's going on."
Variations of the word "underwear" exists to better express various degrees of understanding. For example, if one barely understands a situation, they may say "I rubber-band underwear" showing that they literally - BARELY underwear whereas someone who understands partially may say "I thong underwear."
Variations are left for the user to experiment with.