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The search box at the top of each page is the main tool at this forum (you can learn about the tools and rules here) - it takes you to dictionary definitions and other threads where the words/phrase may have been discussed before. Type cool into the search box and you find this page: cool - WordReference.com Dictionary of English On that page is the definition:
Slang TermsSlang.
great; excellent:What a cool play that was!
socially acceptable, right, or proper: It's not cool to arrive at a party too early.