The Collaborative International Dictionary of English v.0.48:

Yow \Yow\, pron.
   You. [Obs.] --Chaucer.
   [1913 Webster]

The Free On-line Dictionary of Computing (19 January 2023):

Yow!

    /yow/ (From "Zippy the Pinhead" comics) A
   favoured hacker expression of humorous surprise or emphasis.
   "Yow!  Check out what happens when you twiddle the foo option
   on this display hack!"

   Compare gurfle.

   [Jargon File]

   (1995-03-07)


The Jargon File (version 4.4.7, 29 Dec 2003):

Yow!
 /yow/, interj.

    [from “Zippy the Pinhead” comix] A favored hacker expression of humorous
    surprise or emphasis. “Yow! Check out what happens when you twiddle the foo
    option on this display hack!”.