sexy
A person or thing that seems sexually attractive or arousing.
Er findet sie in diesem Kleid besonders sexy.
He finds her especially sexy in this dress.

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A person or thing that seems sexually attractive or arousing.
Er findet sie in diesem Kleid besonders sexy.
He finds her especially sexy in this dress.

As an adjective borrowed from English, 'sexy' is often not declined when it comes after the noun (predicative use). However, when it comes before the noun (attributive use), it is declined like a normal German adjective.
Dieses Auto ist wirklich sexy.
This car is really sexy.
'Sexy' is a loanword from English and belongs to modern colloquial language. It is often used in the media, advertising, and everyday speech, but rather rarely in very formal or official texts.
Die neue Werbekampagne soll jünger und sexyer wirken.
The new advertising campaign is meant to come across as younger and sexier.
In colloquial language, 'sexy' is often used metaphorically for things that are not people. It then means something like 'attractive', 'modern', or 'trendy'. For example, an idea, a job, or a product can be 'sexy'.
Nachhaltigkeit ist das neue sexy Thema in der Wirtschaft.
Sustainability is the new sexy topic in business.
cool
'cool' and 'sexy' can both sound positive in colloquial German and describe something as attractive, modern, or interesting. 'cool' is very broad: it fits clothing, music, ideas, or even a calm person in a stressful situation. 'sexy' often means sexually attractive, but it can also be used for something very desirable or excitingly modern. Learners may confuse the words because both express praise. But 'sexy' is more personal and can quickly sound physical or promotional. 'cool' is usually more neutral and safer in everyday use.

Die Werbung zeigt das Auto als besonders sexy und modern.
The advertisement presents the car as especially sexy and modern.
Deine neue Jacke sieht wirklich cool aus.
Your new jacket looks really cool.