English Grammar police insist thunk is wrong.
What is the correct past tense and past participle?
I think, I thought, I have thought. (“Thought” serves as both past tense and past participle.)
There are a few notable examples of past participles that legitimately use the “u” sound, and that may be what led to people wanting to use “thunk.” Examples:
begin, began, begun
ring, rang, rung
sing, sang, sung
sink, sank, sunk
swim, swam, swum
That’s my guess as to the origin of “thunk,” anyway. Some more-qualified expert may come along and provide more information.
sink, sank, sunk always misleads me into using think, thought, thunk. It’s so odd that the rules change for the words sink and think.
But it’s not “thank.” That’s a different word.
Actually it should be think, thank, thunk. 
Irregular verbs like this are a feature that English shares with German. They have to be memorized because there’s no pattern that applies to enough of them to be of any help.
I’ve never heard “thunk” used except as an obvious joke, or to rhyme with “sunk.” Do people actually use it seriously? If so, in what part of the world (or country)?
ETA: In case anyone cares, verbs changing tense by alternation of the interior vowel is called apophony. I suspect that using “thought” as a past tense of “think” is suppletion, but I’m nowhere near my books so can’t be sure.
I only use thunk for sarcasm or to make a point.
Thank you for clarifying the grammar rules.
You don’t have a question until you can provide a real reference to the long term use of “Thunk” , except in comedic use.
We know its NOT in use at the moment,except as an example common language ERROR.
Yep, just like
fin, fan, fun
slim, slam, slum
Word Ways had an article about these kinds of things some time back. That is, sets of words that followed the pattern of irregular verbs, but were composed of unrelated words. I don’t remember exactly when (in the last couple years, I think) but I’ll hunt it down if anyone is really interested.
Webster recognizes thunk.
Well, anymore. It used to be that they were the pattern. Then -ed came along and stole their jobs. Now, just a few very old words retain the ancient ablaut, although they too at one time stole the job from some other construction. So goes language.
The only time I use “thunk” seriously is when I’m doing onomatopoeia–usually imitating the sound of something going, well, “thunk!” against something else. 
Otherwise, I only use it to be funny, and always to reference that line of Ed’s from Ed, Edd n Eddy: “I think I just thunk!” 
Tongue twister: A skunk sat on a stump. The stump thunk the skunk stunk. The skunk thunk the stump stunk.
Hey. I was just going to post that! I saw it in a Highlights For Children magazine, circa mid-1960’s or so.
Also “drink, drank, drunk” and maybe “stink, stank, stunk” although I’m unsure about the latter and don’t often hear it in the perfect tense anyway.
But not “cling, clang, clung”, and “hang” is the present tense when it ought to be the past historic of “hing”
People never want to use “drunk” correctly. “Hey! all the milk has been drunk!” People always want to say “drank” in sentences like that. I think it’s because “drunk” is so stigmatized as meaning intoxicated, that people don’t want to use it for anything else.
I wonder if the reason executed people have been “hanged,” not “hung,” is to prevent a similar thing from happening to “hung.” Also, a “hanged man,” and a “hung man” (not to mention a hangman) are very different things.
Fling, flang, flung-- I wouldn’t have believed this one, except I saw “flang” in some published stage directions of an O’Neill play. Now, I know published stage directions are just taken from the assistant director’s notes from the original production, but still.
Also, “Song she sang to me / Song she brang to me / Words that rang in me / Rhyme (sic) that sprang from me.” Even if it were bring, brang, brung, shouldn’t that line be “The song she sung to me, the song she brung to me…/rung…/sprung”? Or maybe even “had sung.” Also, “rhymes”
It’s a disaster of a song.
No, “sang” and “sprang” are past historic and should be used in this context so “brang” would be. It would be “Song she has sung to me” if you used the past perfect.
But let’s not tense up about this.
If you look at the whole song, it really ought to be, IMO, “the song she had sung to me,” because he talks about the singing in the past, and then something that also happened in the past, but after, and as a result of, the singing. I don’t know if there’s a good way to write this song, and it ought to go in the scrap heap.