It's the shortening of "subliminal tweet" which is directly
referring to a particular person without mentioning
their name or directly mentioning
them and it basically indicates that the tweet in which the hashtag is used is a subliminal tweet.
Basically, it's talking about
someone behind their back but sort
of in their face on Twitter!
Man, I wish this
stop subtweeting about me and just say what
she has to say directly. Triflin'!
I wish #oneofmyfollowers would
stop #subtweeting and just come out with what
he wants to say! Ugh!