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1.
Describes when someone on Twitter over-uses hashtag's in their tweets.

A person would use it to inform some-one else that they have used too many hashtags in a tweet.

Hashslags are often new to twitter, just having moved from facebook, and get over excited easily by the fact that they can now hashtag.
*On Twitter*

Person 1: #MeganFox is a #sexy #beast

Person 2: dude, what's with all the hash-tagging?

Person 1: #calmdown it's #twitter man! #becool #chillax

Person 2: *cough* hashslag

Person 1: ¬_¬

Person 2: ...
by KatiieAbs April 03, 2012
 
2.
A person who uses excessive amounts of hashtags on their Instagram/Twitter posts. These hashtags include but are not limited to writing each element of a sentence with a hashtag in front of it. Hashslags frequently commit this atrocity to receive as many followers/likes/retweets/loves as possible from other social networkers.
"Pic of me in the toilets of a club with ma friend" #me #in #the #toilets #girl #eyes #boobs #legs #heels #friend #love #club #nightout #nightclub #happy #pink #selfie #yolo #hehehe #.

-you use too many hashtags. #hashslag
by H-bomb May 19, 2013
 
3.
Someone who uses excessive hashtags for "likes" or to obtain followers on social media.
She posts like fifty pics a day. She's such a hashslag, it's pathetic.
by Scotty J July 29, 2014