First attested in the Gugeupganibang eonhae (救急簡易方諺解 / 구급간이방언해), 1489, as Middle Korean ᄇᆞᆰ〯쥐〮 (Yale: pǒlk-cwúy). 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. 
- (SK Standard/Seoul) IPA(key): [ˈpa̠(ː)k̚t͡ɕ͈ɥi] ~ [ˈpa̠(ː)k̚t͡ɕ͈y]
- Phonetic hangeul: [박(ː)쮜]
- Though still prescriptive in Standard Korean, the great majority of speakers (in both Koreas) no longer distinguish vowel length.
|Revised Romanization (translit.)?||bagjwi|
- 밝쥐 (bakjwi) (archaic; literally "bright (eyed) rat")
- 박쥐구실 (bakjwigusil, “opportunism”)
- 박쥐우산 (박쥐雨傘, bakjwiusan, “umbrella (made of cloth)”)
- 박쥐족 (박쥐族, bakjwijok, “nocturnal”)
- 날짐승 (naljimseung): winged animal, literally "flying animal"
- 낮잠 (natjam): nap, literally, "daytime sleep"
- 메아리 (meari): echo
- 밤나들이 (bamnadeuri): night tour, literally, "night going out and coming in"
- 밤눈 (bamnun): night vision, literally, "night eye"