Who is a koston sie?

Answer 1

koston is not a German word. Kosten sie could mean taste them or taste this! in English. The exact meaning would depend on whether sie is capitalized or not, and the rest of the sentence.

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Assuming that the correct spelling is kosten, then there can only be one meaning in German. Kosten Sie would be said by a German as a formal invitation to taste something, it simply means (have a) taste! e.g.

Schmeckt das (gut)? - Ja, kosten Sie - Does that taste nice? - Yes, have a taste

Taste them in German would be:

koste sie (informal)

kosten Sie sie (formal)

Taste this! in German would be:

koste dies (informal)

kosten Sie dies (formal)