Having a random flashback of the events that took place while under the effect of some dank marijuana. Usually happens while sober, and the memory suddenly hits you out of the blue, even though you can't remember the rest of the night. Sometimes it's possible to piece together a whole night by putting together everyone's hashbacks.
"Dude...and then you tried to do a backflip!"
"Holy shit! And I faced it right on the curb too, right before the cops showed up! I remember! Hashback!"
When somebody elsejacks your or others hashtags to promote there twitter page or product
Example of Hashjacking:
Omg check out this cool thing or a tweet full of hashtags
#NYCMarathon
#PrimarySchoolMemories
#Secondaryschoolmemories
#IveAlwaysWantedTo
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.