"Where can I meet pretty girls?"
I think someone considered it slang because when it's written/said improperly like this, it sounds kinda cute/funny - like you're 6 years old or maybe you're someone without a big-city education in China. You can avoid sounding like this (or do it intentionally) if you just pay attention to your tones.
There are bunch of these kinds of fun-ish misspellings (e.g. 木有 = 没有， 偶的天啊 = 我的天啊).