Weepinbell acid is a remarkably corrosive substance, which dissolves many materials with such speed and effectiveness that it is used throughout the pokemon world as an alternative to dynamite. Developed as a means of killing and digesting prey such as Rattata or Caterpie, it is capable of dissolving even Onix and other rocks. Weepinbell leak acid from their eyes frequently, in a process which has evolved like a Growlithe puppy's eyes to elicit sympathy from humans, and this acid frequently seeps deep into the earth's crust.
This acid has thankfully not been effectively weaponized, for no container has been found which can both hold the acid and not cause it to lose potency, save for the Weepinbell themselves. And Weepinbell are not an especially powerful pokemon; they can only throw their acid a short distance, are easily knocked out, and humans and larger pokemon require the continuous application of this acid to be killed in the first place.
This is not to say that living creatures are immune. Although Weepinbell themselves produce a neutralizing fluid, it is an imperfect one, and they produce less and less as they age. If they are unable to evolve soon enough into Victreebel, a Weepinbell will die from its own acid, being dissolved from the inside out.