1. [n]  the act of hitting vigorously; "he gave the table a whack"
2. [v]  hit hard; "The teacher whacked the boy"

1. \Whack\, v. t.
   To divide into shares; as, to whack the spoils of a robbery;
   -- often with up. [Slang]
   
   

2. \Whack\, n.
   A portion; share; allowance. [Slang]
   
   {Out of whack}, out of order. [Slang]
   
   

3. \Whack\, v. t. [imp. & p. p. {Whacked}; p. pr. & vb. n.
   {Whacking}.] [Cf. {Thwack}.]
   To strike; to beat; to give a heavy or resounding blow to; to
   thrash; to make with whacks. [Colloq.]
   
         Rodsmen were whackingtheir way through willow brakes.
                                                  --G. W. Cable.
   
   

4. \Whack\, v. i.
   To strike anything with a smart blow.
   
   {To whack away}, to continue striking heavy blows; as, to
      whack away at a log. [Colloq.]
   
   

5. \Whack\, n.
   A smart resounding blow. [Colloq.]
   
   

According to arch-hacker James Gosling, to "...modify a program with no idea whatsoever how it works." (See whacker.) It is actually possible to do this in nontrivial circumstances if the change is small and well-defined and you are very good at glarking things from context. As a trivial example, it is relatively easy to change all "stderr" writes to "stdout" writes in a piece of C filter code which remains otherwise mysterious.

[jargon file]