thank-you - Wiktionary, the free dictionary

• thankyou
• thank you

Compound of thank +‎ you.

thank-you (plural thank-yous)

1. An expression of gratitude.

   I must offer a big thank-you to my agent for getting me this job.

2. (as a modifier before a noun) That expresses thanks.

   a thank-you letter

• Thank-you is a noun. The interjection thank you is always spelled as two words.