Sneaked vs. Snuck
Until just over a century ago, if you did the thing we called sneak, but you did it yesterday, you would have sneaked.
This follows the pattern all English verbs use to form their past tenses. Today I walk, yesterday I walked. Today I bake, yesterday I baked. Today I peek, yesterday I peeked. So for the longest time it was Today I sneak, yesterday I sneaked.
But sometime in the late 1800s, people started replacing sneaked with snuck. Today I sneaked, yesterday I snuck. And here’s the weird part: No one knows why, according to the blog at Merriam Webster’s Collegiate Dictionary. People just started using it.
That kind of thing happens in language all the time. A lot of our regular verbs were once irregular, but people started applying the "ed" formula we see in walked, baked, and peeked. It’s a natural process.
But snuck is different for one very important reason: No other English verb follows this pattern. So it’s like people who needed to use the verb “peek” in the past tense began to eschew "peeked" in favor of "puck." No one knows where that “uck” ending came from or why.
All they know is that, in the past 120 years, “snuck” has gained solid legitimacy as an alternative to “sneaked.” Merriam Webster’s even reports that “over the past 120-odd years, ‘snuck’ has become by some estimations the more common past-tense form. Some people object to the sneaky upstart – especially speakers in British English – but it appears regularly and without commentary in respected publications on both sides of the pond.”