By The Way I Think I'm Falling in Love With You. It's the first letter of every word in that sentence. It's a way to tell someone how you feel without actually saying it.
Girl:"Some guysaid i was ugly."
Boy:"Don't listen. I think ur beautiful."
Girl:"Aw, thnx. btwitifilwy."
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi