The Full form of BTW is By The Way. BTW is shorthand for By The Way. It is typically used by a person to change the subject during a conversation. It can be used as a post amble or a preamble to signify the change of subject. For example: BTW I like your watch (Here, BTW is a preamble, i.e., before the text), and I like your watch BTW (Here, BTW is a post amble, i.e., after the text). Of note, BTW is also occasionally used to mean “back to work”. “By the Way” is the most common meaning for BTW on WhatsApp, Snapchat, Twitter, Facebook, and Instagram.
It is also become a popular chat abbreviation that is used in online communication such as online chat, e-mail, and instant messaging. So, if someone wants to change the subject during a conversation immediately, he can use this Acronym in his chat, email, text messages, etc.