Ah, I presume you mean you looked up thank-you, which is an existing noun but not the same thing at all as the common idiom thank you. Merriam-Webster tells us: Full Definition of THANK-YOU: a polite expression of one's gratitude. Origin of THANK-YOU. from the phrase thank you used in expressing gratitude First Known Use: 1792 More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.