- simple past tense and past participle of
In casual use, found even in careful speech, interchangeable with shrank; in careful formal use, only used for past participle "I have shrunk", while shrank is used for the past tense "I shrank". Compare sank/sunk. The inconsistent usage is due to the fact that shrink is a Germanic strong verb, hence conjugated via ablaut (change of vowel rather than adding -ed), but these are irregular in modern English. The past tense "shrunk" is derived from the Old English plural past "scruncon". The same form is found in other past tenses, such as "slunk".
The 1989 movie Honey, I Shrunk the Kids (formally: Honey, I Shrank the Kids or Honey, I've Shrunk the Kids) is an example of the prevalence of the casual form.
Note that in the 1844 translation of the Septuagint by Sir Lancelot Charles Lee Brenton, the form "shrank" is used in IV Maccabees 14:4 ("None of the seven youths turned cowardly, or shrank back from death", singular subject), whereas "shrunk" is used in I Maccabees 3:6 ("Wherefore the wicked shrunk for fear of him, and all the workers of iniquity were troubled, because salvation prospered in his hand", plural subject).
The preferred form when used adjectivally is "shrunken".