この単語は初めて見たので、意味が分かりません。教えて！って英語でなんて言うの？

ーI've never seen this word before. I have no idea what it means. Could you tell me please. 「この単語を今までに見たことがない。その意味が全然わからない。教えてください。」 初めて見たは「見たことがない」I've never seen ... と言えます。 ーThis is the first time I've ever seen this word. I'm not sure what it means. Could you tell me? 「この単語を見たのは今までで初めてです。意味がわかりません。教えて。」 This is the first time 「これが初めて」という言い方です。 ご参考まで！

By using the expression "not familiar with this," one is stating it is not clear or has not been experienced prior to this time. This request can also be expressed as a statement that tells the person to describe the word and then giving the reason why it needs to be described. The second example sentence does that.

When you use the phrase "I have never come across this word before, it means that you have never heard of the word before, during your time learning/speaking the English language, it's a word that hasn't come up for you before, it is completely new for you.

cobaさん ご質問どうもありがとうございます。 様々な表現があると思いますが、いくつか自然な表現を紹介します。 - I've never seen this word before. What does it mean? - Never seen that one before. What does it mean? 「What does it mean? 」の後に、英語では「教えて！」など言わなくてもよいかもしれません。なぜなら、「分からないから教えてね～」というお願いは、はもうすでに暗黙的に存在するからです。 お好みに合わせて使い分けてみてください。 ご参考にしていただければ幸いです。

I don't know what this word means. I've never heard of it. Could you tell me? この単語の意味がわかりません。一度も聞いたことがないです。教えてくれますか？ 初めて見た（おそらく文書などで）ということだと思いますが、それは既に回答があるので、「聞いたことがないです」という方を挙げてみました。

It's common to say that you haven't heard that word or seen that word before which that in itself shows that you don't know the meaning but it would be polite to ask what it means maybe also adding please.

If you want to ask your teacher what a particular word means then you can simply ask, 'What does that mean?' or 'What does that word mean?' To give your question a bit more context and understanding you could say, 'I'm unsure what that word is, can you let me know the meaning?' This shows your teacher that you have not come across this word before and you want to know what it means.