A somewhat old-fashioned or literary term for an uncultured, rough-mannered person, often from the countryside. Can be used humorously or insultingly.
He's a rustic who just came to the city.
Don't expect manners from that boor.
田舎者 is a more common and neutral term for a country person, while 無骨者 emphasizes rough, uncouth behavior.
無骨 is an adjective/noun meaning 'rustic' or 'boorish'; 無骨者 is the person who embodies those traits.
Standard kanji spelling. 無骨 (rustic, boorish) + 者 (person).
Variant spelling using 武 (martial) instead of 無 (without). Less common but attested.
Compound of 無骨 (boorish, rustic) and 者 (person). The exact historical derivation of 無骨 is uncertain, but it is conventionally associated with the idea of lacking refinement.