This is the 825th most frequent Japanese word.
ワン
"ワン" (wan) is the sound of a dog's bark, like "woof" in English.
Here, 'ワン' is part of 'ワンピース' (wanpiisu), which means 'dress' in Japanese.
彼女はワンピースが好きです。
She likes dresses.
'ワン' is an onomatopoeic representation of a dog's bark in Japanese.
犬は『ワン』と鳴きます。
Dogs bark 'woof'.
'ワン' in 'ワンマン' (wanman) derives from English meaning 'one man,' referring to something operated by a single individual, here a bus.