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
ぶにん
shorthandedness
むりやり
forcibly
むり
impossible
むちゃ
rash
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