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1.
A hashtag on social media platforms such as Twitter or Instagram that is unnecessary or ridiculous, and is therefore colloquially defined as being 'gash'.

As a general rule of thumb, people who gashtag don't fully understand the concept of hashtagging - that by placing the # symbol in front of words designates them as search terms for other users.
Generic tweet: "OMG Made In Chelsea is on #imsoinlovewithspencermarrymeplease" - this is a gashtag because no one is going to type that in to a twitter search and clicking on it is unlikely to show any other tweets because the tag is so ambiguous

Or

New photo on Instagram: New dress #selfie #lipstick #and #lashes - in this case 'and' is the gashtag, it is unnecessary because 'and' is not a term most users will want to search for, and if it is, they probably weren't looking for a selfie from a pouting teenage girl.
by Marky14Mark June 19, 2013
3 2
 
2.
Expressing your appreciation of a hottie on twitter via the use of a hashtag
John: Look at that girl's display picture

Adebayo: I'm gashtagging that #gashtag
by Adebayo Akinfenwa February 01, 2012
6 6