tmb is an acronym for 'text me bitch'. It is used to text your friends when they have clearly gottendistracted from what is most important to them, texting you back and entertaining you when you're bored.
Colleen: Blah blah blah Zane!
Zane: Blah blah blah Colleen!
30 minutes with no response...
Zane: ...tmb! :P
Colleen: Blah blah sorry I fell asleep blah!