There are many terms to describe a stupid person, depending on the reason why youo think they are stupid, whether it is behaviour, intelligence (or lack of it!).
Here are a few examples:
idiot, imbecile, moron, birdbrain, airhead, muppet, dunce just to mention a few.
All of these nouns are interchangeable so you could swap them into any sentence and it would still make sense, for example:
How did you fail that test?! You are such a(n) moron/airhead/dunce.
愚かな人を表す表現はたくさんあります。これは、どうして愚かだと思うのかによります、行動なのか知能なのか。
以下は、そのいくつかです。
idiot, imbecile, moron, birdbrain, airhead, muppet, dunce
これらの名詞は全て "interchangeable"（置き換え可能）です。つまり、置き換えて使っても意味が通じます。
例えば：
How did you fail that test?! You are such a(n) moron/airhead/dunce.
「どうやったらあのテスト落ちるの。本当にバカだよね」
A person that is stupid is usually called an "idiot", a "fool"or a "numb-skull".
There are many other terms that may be used but the above are only a few. You can use these in a sentence in the following ways:
-Sam is an idiot. He let his gorgeous girlfriend go.
-Sam is going to regret divorcing his wife. He was a fool to let a good woman go.
-Sam is a numb-skull. He makes terrible choices.
愚かな人のことは、よく "idiot" や "fool" "numb-skull" といいます。
他にもたくさんの言い方があります。上記はそのほんの一部です。
文の中では次のように使えます。
-Sam is an idiot. He let his gorgeous girlfriend go.（サムはバカだ。最高の彼女を手放してしまった）
-Sam is going to regret divorcing his wife. He was a fool to let a good woman go.（サムは離婚したことを後悔するよ。素晴らしい女性を手放して、愚かだよ）
-Sam is a numb-skull. He makes terrible choices.（サムはバカだよ。判断力がない）
These words are all related to the word stupid. They are what we call synonyms. Synonyms are words that have the same or similar meaning to the original word. Other synonyms include: moronic, imbecile, obtuse, gullible, naive; informal-thick, thick as two short planks, dim, dumb, dopey, dozy, crazy, barmy, cretinous, birdbrained, pig-ignorant, bovine, slow on the uptake, soft in the head, brain-dead, boneheaded and dumb-ass
3例とも "stupid" という単語に関連した表現です。これらは "synonyms"（同義語）です。"synonyms" とは同じか、あるいは似た意味を持つ単語をいいます。
他の "synonyms" には、次のようなものがあります。
moronic, imbecile, obtuse, gullible, naive; informal-thick, thick as two short planks, dim, dumb, dopey, dozy, crazy, barmy, cretinous, birdbrained, pig-ignorant, bovine, slow on the uptake, soft in the head, brain-dead, boneheaded, dumb-ass
When someone acts in a silly or stupid way we often use the word 'idiot' to describe them
To be foolish or a 'fool' means to act in a joking or funny way and be silly: 'You are a fool!'
愚かな行動をする人のことは、よく 'idiot' と表されます。
'foolish' あるいは 'fool' は、おかしな行動をする愚かな人をいいます。
'You are a fool!'
（あなたはバカだ）
There are so many words in the English vocabulary that refers to a person who is stupid or foolish. Some of these are much ruder than others. How rude this is also depends on the tone, pitch, and emphasis you use with your voice. All three words mentioned here could be used in a friendly way if you used a high pitched voice and it was accompanied with a friendly laugh.
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