無蓋 is read as むがい (mugai) and means open.
むがい
mugai
むがいかしゃ
open freight car
むがいしゃ
むじん
shorthandedness
むちゃ
rash
むりやり
forcibly
むり
impossible
むりょう
free (of charge)
ぶじ
good health
むしょく
without an occupation
むごん
silence (not speaking)
むせん
radio (communication)
むよう
unnecessary