Why was "tantrum" trending last week?
1833, from sneak (v.) + -y (2). Related: Sneakily; sneakiness. Sneaky Pete "cheap liquor" is from 1949.
(also sneak preview): After a sneak in Chicago they decided to shelve it
To show a movie unexpectedly to an audience in order to assess its appeal: We sneaked it in several cities (1960s+ Movie studio)