move or go in a furtive or stealthy way

I sneaked out by the back exit

(especially in children's use) inform an adult or person in authority of a companion's misdeeds; tell tales

she sneaked on us

(especially in children's use) someone who informs an adult or person in authority of a companion's misdeeds; a telltale

Ethel was the form sneak and goody-goody

acting or done surreptitiously, unofficially, or without warning

a sneak thief

a sneak preview