Green’s Dictionary of Slang

a pleasurable feeling.

1997		J. Whedon Buffy the Vampire Slayer 3 Nov. [TV script] If Angel’s doing something wrong [...] it gives me a happy.
1999		USA Today 23 Dec. 04D: Among other terms: [...] ‘a happy,’ as in ‘Ice cream gives me a happy’ (pleasure); and ‘freaksome,’ as in ‘That’s fairly freaksome’ (bizarre).
2020		G. Armstrong Young Team 15: The warnin signs mean huhin tae him, compared tae the six bottles ae happy.