
knackig
crisp
Describes foods that are fresh, firm, and make a sound when you bite into them.
Der Apfel war saftig und knackig.
The apple was juicy and crisp.
crisp / toned / concise
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crisp
Describes foods that are fresh, firm, and make a sound when you bite into them.
Der Apfel war saftig und knackig.
The apple was juicy and crisp.

toned
Colloquial for a firm, well-trained, and attractive body or body part.
Durch das viele Training hat er einen richtig knackigen Hintern bekommen.
From all the training, he got a really toned butt.

concise
Short, succinct, and to the point.
Er fasste die Ergebnisse in drei knackigen Sätzen zusammen.
He summarized the results in three concise sentences.

biting cold
Describes very cold, but usually dry and clear, cold weather.
Bei der knackigen Kälte von minus zehn Grad gefror der See.
In the biting cold of minus ten degrees, the lake froze over.

tough
Colloquial for a difficult, demanding task or problem.
Die letzte Frage in der Prüfung war eine richtig knackige Nuss.
The last question on the exam was a really tough nut to crack.
The word is derived from the sound 'knack' (crack/snap). The basic idea is something firm, fresh, or punchy. This idea connects many of the meanings: an apple that cracks when you bite into it, a short and pointed statement, or a 'hard' task.
Ich mag Äpfel, die beim Reinbeißen richtig knacken.
I like apples that really crack when you bite into them.
Most often, 'knackig' (crisp) describes fresh foods. Typical combinations are 'knackiger Salat' (crisp salad/lettuce), 'knackiges Gemüse' (crisp vegetables), or 'knackige Äpfel' (crisp apples).
Zum Grillen gibt es Würstchen und einen knackigen Salat.
For the barbecue, there will be sausages and a crisp salad.
When 'knackig' (toned) describes a body or body parts, it is very colloquial. It is used as a compliment for a firm, athletic appearance.
Sie macht viel Sport für einen knackigen Bauch.
She does a lot of sports to get toned abs.
In connection with language, 'knackig' (concise) means short and to the point. A very common phrase is 'kurz und knackig' (short and sweet).
Bitte fasse deine Idee kurz und knackig zusammen.
Please summarize your idea briefly and concisely.
To describe cold, 'knackig' is used for dry, strong frost. For example, people say 'knackige Kälte' (biting cold) or that it is 'knackig kalt' (bitterly cold).
Zieh dich warm an, draußen ist es knackig kalt.
Dress warmly; it's bitterly cold outside.
A difficult task is often colloquially called a 'knackige Nuss' (a tough nut to crack). The adjective alone can also be used to describe a demanding problem.
Die letzte Matheaufgabe war eine wirklich knackige Nuss.
The last math problem was a really tough nut to crack.
crispy / crunchy
Both words describe foods that make a sound when you bite into them. „Knackig“ usually emphasizes freshness and a firm texture, while „knusprig“ refers especially to the hard, dry crust of baked or fried food. Use „knackig“ for fresh vegetables and „knusprig“ for the crust of a dish.

Der Salat schmeckt knackig, weil das Gemüse frisch und fest ist.
The salad tastes crisp because the vegetables are fresh and firm.
Das Brot schmeckt knusprig, weil seine Kruste im Ofen hart und trocken geworden ist.
The bread tastes crispy because its crust became hard and dry in the oven.