Wikipedia:Strong

Wikipedia's policy has long held that "decisions on Wikipedia are primarily made by consensus, which is accepted as the best method to achieve Wikipedia's goals" Consensus is determined through discussion. Sometimes this discussion looks an awful lot like a vote, but it isn't supposed to be. Often, users will attempt to strengthen their position by adding the word "strong" to their bolded support or oppose comment preceding their rationale. If they are really fired up they might add very strong or even strongest possible.

Doing this does nothing to actually strengthen your position or influence what consensus comes out of the discussion, especially if it is all you have to say. A clear and logical statement of your position, with or without a bolded comment prefacing it, is what makes your argument strong.

What about "weak"?
Adding the "weak" modifier is a bit different as it self-identifies your position as weak, meaning it really isn't a big deal to you either way. You could just say as much, but it is a convenient shorthand.