The Collaborative International Dictionary of English v.0.48:

Whacker \Whack"er\, n.
   [1913 Webster]
   1. One who whacks. [Colloq.]
      [1913 Webster]

   2. Anything very large; specif., a great lie; a whopper.
      [Colloq.] --Halliwell.
      [1913 Webster]

WordNet (r) 3.0 (2006):

whacker
    n 1: something especially big or impressive of its kind [syn:
         whacker, whopper]

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

whacker
 n.

    [University of Maryland: from hacker]

    1. A person, similar to a hacker, who enjoys exploring the details of
    programmable systems and how to stretch their capabilities. Whereas a
    hacker tends to produce great hacks, a whacker only ends up whacking the
    system or program in question. Whackers are often quite egotistical and
    eager to claim wizard status, regardless of the views of their peers.

    2. A person who is good at programming quickly, though rather poorly and
    ineptly.


The Free On-line Dictionary of Computing (30 December 2018):

whacker

   [University of Maryland: from hacker] 1. A person, similar
   to a hacker, who enjoys exploring the details of
   programmable systems and how to stretch their capabilities.
   Whereas a hacker tends to produce great hacks, a whacker only
   ends up whacking the system or program in question.  Whackers
   are often quite egotistical and eager to claim wizard
   status, regardless of the views of their peers.  2. A person
   who is good at programming quickly, though rather poorly and
   ineptly.