No, not the supposed sound that ghosts make, but an ancient word for "warrior", "martial" and even "marital arts." If you are an avid reader of Japanese or Chinese fiction, especially those set in the medieval times, you will have encounter whenever a hero, wanting to underline his prowess, exclaiming that his 武 (bu) is matchless.
The term is - as are most one-kanji words - notoriously hard to translate.
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