If the phrase is used to describe a strong or angry person, "mapanga" evokes the image of a dangerous cave (home to wild animals), and "mukha" refers to the facial expression.
Less common, but in proverbs, "mapanga" represent hidden dangers (like a cave hides beasts), and "mukha" is the outward face. The phrase implies someone who looks harmless but is dangerous. mapanga na mukha in english top