
köstlich
delicious
Tasting very good.
Vielen Dank für das Abendessen, es war wirklich köstlich!
Thank you very much for dinner; it was truly delicious!
delicious / delightful
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delicious
Tasting very good.
Vielen Dank für das Abendessen, es war wirklich köstlich!
Thank you very much for dinner; it was truly delicious!

delightful / hilarious
Very entertaining or amusing, often in a subtle, witty way. It is frequently used in the phrase 'sich köstlich amüsieren' (to have a wonderful time / to be highly amused).
Wir haben uns köstlich über seine witzigen Geschichten amüsiert.
We were highly amused by his funny stories.
Careful, 'köstlich' (delicious/delightful) is a false friend! It looks like the English word 'costly' (teuer), but it means 'very tasty' or 'very amusing.'
Der Kuchen war köstlich, aber zum Glück nicht teuer.
The cake was delicious, but fortunately not expensive.
The word 'köstlich' (delicious/delightful) is historically related to 'Kosten' (costs). It originally meant 'precious' or 'valuable,' from which the modern meaning 'especially good' developed.
Früher galten Gewürze als eine köstliche und teure Ware.
In the past, spices were considered a precious and expensive commodity.
In the meaning 'amusing,' 'köstlich' (delightful/hilarious) is very often used in the fixed expression 'sich köstlich amüsieren' (to have a wonderful time / to be highly amused). It means that someone is having a lot of fun or laughing a lot.
Das Publikum amüsierte sich köstlich über den Komiker.
The audience was highly amused by the comedian.
The two meanings are easy to distinguish from context. If it is about food and drink, 'köstlich' (delicious) means 'very tasty.' If it is about entertainment, jokes, or stories, it means 'very amusing.'
Der Wein zu diesem Gericht ist köstlich.
The wine with this dish is delicious.
delicious
Lecker and köstlich both say that something tastes very good. Lecker sounds everyday and somewhat colloquial. Köstlich usually expresses stronger praise of the taste and can sound more elevated; it therefore works well in especially enthusiastic or stylistically refined descriptions.

Die Restaurantkritikerin beschreibt dasselbe Curry als köstlich.
The restaurant critic describes the same curry as delicious.
Das Curry ist echt lecker, sagt Mia zu ihrem Freund.
“This curry is really delicious,” Mia tells her boyfriend.