1. The very means of slapping someone (or something) in the face (or elsewhere) with ones penis. derived from the sound it makes when contact is made.
1. WHOPACHA! (*screamed whilst performing the act.)
2. That bitch is gonna cop a whopacha in the face!
3. that n00b just got whopacha'd
4. OMGWTFBBQ! i just got whopacha'd