Is thunk a word?
Thunk is not really a word, but you will find it used for comic effect (e.g., “Who’da thunk it?”). The past tense and past participle of think is thought.
Think is an irregular verb and doesn’t add “-ed” to form the past tense (e.g., “There’s no way he thought that was OK!”) or past participle (e.g., “Hamish had thought carefully about the decision”).
The QuillBot Grammar Checker can help ensure you’re using the past tense of think correctly.