Of native Korean origin. Shift from earlier 밝쥐(bakjwi), itself a compound of 밝-(bak-, “bright”, probably implying “bright-eyed”) + 쥐(jwi, “rat”). In the past, Korean people thought bats had bright eyes because they did not know that they could fly in dark caves using ultrasound.  Another possible theory is that 박쥐 might have come from 븕(red)+쥐(rat). This is because 븕 has two different meanings: one is bright and the other is red.