Sneak (to move stealthily): sneak > sneaked > sneaked
The past and perfect past of sneak are always sneaked. The nonstandard snuck sneaked into the language in the 1900s, growing in popularity in the 2000s. How it sneaked into our language is unknown—no other -eak or -eek word forms the past tense with -uck. While snuck has become more popular in American English, it remains nonstandard. Prefer sneaked in all situations.