ばか・馬鹿・莫迦baka[1]atamadaka (head-high)

1. nounidiot; moron; fool
   こんな荒あれ模様もようの日ひに外出がいしゅつするなんて君きみは馬鹿ばかだよ。↗

   You are an idiot to go out in this weather.
2. na-adjectivestupid; foolish; dull; absurd; ridiculous
   なんという馬鹿ばかなことだろう。↗

   What a piece of folly!
3. noun, na-adjectivetrivial; insignificant; disappointing
4. noun, na-adjectivemalfunctioning; defective; losing sensation
5. noun, noun (prefix)incredibly; unusually; exceptionally
6. noun, noun (suffix), na-adjectivefervent enthusiast; nut; person singularly obsessed with somethingusu. in compounds
7. nounMactra chinensis (species of trough shell)

• SynonymアホCasualInsult common in Kansai meaning fool; many Kansai people find 「アホ」 friendlier than 「馬鹿」, but how offensive it is still depends on the relationship and context.
• Synonym間抜けまぬけNeutralHighlights carelessness—slipping up at the crucial moment or lacking sense; 「馬鹿」 stresses being dim-witted.
• Synonym愚ぐWrittenWritten word for foolishness, also a humble way to refer to oneself; mostly in compounds and set phrases, never used to insult in speech.
• Synonym愚かおろかWrittenA na-adjective for foolish, thoughtless behavior, leaning written; 「馬鹿」 is more colloquial and carries an insulting tone.
• Synonym白痴はくちCasualSame word as the earlier 「戯け（たわけ）」 entry, also written 「白痴」; when read はくち it is a different word.
• SynonymのろまCasualSomeone slow-moving and slow on the uptake; the emphasis is on slowness, while 「馬鹿」 stresses stupidity.
• SynonymめちゃくちゃCasualMeans a mess, absurdity, or something extreme; close to 「馬鹿」 in the sense of absurd or outrageous.
• SynonymとんまCasualA fool who is rash or slow to react, with a comic flavor; rare today. 「馬鹿」 covers stupidity in general and is far more widely used.
• Synonymうつけ者うつけものWrittenArchaic term for someone who cannot grasp reason and acts foolishly, seen mostly in period dramas; in daily speech say 「馬鹿」.
• SynonymたわけCasualRead たわけ; an old-fashioned insult for a fool or for fooling around, common in period dramas; in everyday speech use 「馬鹿」.
• Synonym愚鈍ぐどんWrittenWritten word for a slow, dim mind, pejorative in itself and mostly seen in prose; 「馬鹿」 is a spoken insult.
• SynonymうすばかCasualSomeone not very bright, with contempt; it can serve as a harsh insult and is no politer than 「馬鹿」.
• Synonym馬鹿者ばかものNeutralAn insult for a fool or idiot, stronger than 「馬鹿」 and often used to rebuke someone.
• Synonymたわけ者たわけものCasualOld-fashioned insult for a fool, familiar from period dramas; in daily speech say 「馬鹿」 or 「馬鹿者」.
• SynonymばかたれCasualAn intensified insult for a complete idiot, with regional coloring; stronger than 「馬鹿」.
• Synonym愚人ぐじんWrittenWritten word for a foolish person, mostly in maxims or prose; rarely heard in everyday conversation, where 「馬鹿」 is the usual word.
• Synonym痴人ちじんWrittenWritten word for a foolish person, found mainly in literature; not used in everyday speech—「馬鹿」 will do.
• SynonymばかやろうCasualA violent insult meaning bastard or idiot, also usable as an exclamation; far rougher than 「馬鹿」.
• Synonym愚者ぐしゃWrittenWritten word for a fool, the counterpart of 「賢者」, and also the name of the Tarot Fool card; not used to insult in daily life.
• Synonym愚昧ぐまいWrittenWritten word for ignorance and inability to grasp reason, often describing groups or eras; 「馬鹿」 usually targets individuals.
• Synonym愚蒙ぐもうWrittenRare written word for ignorance; in everyday use say 「愚か」 or 「馬鹿」.
• Synonym愚か者おろかものNeutralA foolish person; slightly more formal than 「馬鹿」 and often used in scolding.
• Synonym愚物ぐぶつWrittenWritten word for a fool with a contemptuous edge, mostly in prose; for spoken insults use 「馬鹿」.
• Synonym痴愚ちぐWrittenWritten word for stupidity, once also a medical term for a grade of intellectual disability, no longer used medically.
• Synonym大愚たいぐWrittenGreat folly or a great fool; also a self-deprecating pen name, as when the monk Ryōkan styled himself 「大愚」; not used in daily life.
• Synonym下愚かぐWrittenArchaic written word for an extremely stupid person, also used humbly of oneself; in daily speech say 「大馬鹿」.
• Antonym賢者けんじゃWrittenAntonym: a sage or wise person, with a written flavor; the opposite of the fool that 「馬鹿」 denotes.
• Antonym利口りこうNeutralAntonym: clever and bright, also well-behaved of children and pets; the opposite of 「馬鹿」.

For everyday 'fool' or 'stupid,' 「馬鹿」 is the default. In Kansai, 「アホ」 is the usual insult and often feels friendlier. For careless slip-ups choose 「間抜け」; in written prose use 「愚か」. For a stronger rebuke, escalate to 「馬鹿者」, 「馬鹿たれ」, or the rough 「馬鹿野郎」. Most other terms—「愚鈍」, 「愚人」, 「愚者」, 「痴人」—are literary or archaic words for fools seen in prose or period dramas, not daily speech; 「鈍間」 stresses slowness, 「滅茶苦茶」 absurdity or excess. Antonyms: 「賢者」 (sage), 「利口」 (clever).

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. 中国ちゅうごくにおける、ある種しゅの鹿しかの呼称こしょう。日本にっぽんでいうアカシカ。漢方薬かんぽうやくの鹿茸ろくじょうを採取さいしゅする。