It is colloquial because it is a current trend, more so with the younger generation. It's meant to sound cute, and ironic since hashtags aren't meant to be voiced, but written rather.
The real word here is
Hash refers to the # symbol
tag part of the phrase is the subject here. The act of tagging things (in this case, words) using the hash symbol.
Why the # hash?
Because, in social media, discourse communities, the hash symbol is not typically used, unless you're referring to something in a series of numbers and want to specify one in particular.
#9 is the winning horse
From a programming stand point, the hash was just chosen arbitrarily by whomsoever came up with the idea in the first place. The creator could have just as well had used
!@$%^&*()_+-= or more
In programming, there is something called regular expressions which allow you to capture specific words in a set of words with certain conditions.
I would like a house, or a boat, or a houseboat.
Using regular expressions, someone can extract the three options with a creatively crafted pattern that I will not go into, but for explanations sake.
Let's take this
Cookies are good #goodcookies.
Using regular expressions, one can capture the
hashtag.. AND omit the period.
Sometimes at dark #inthedark aliens visit my room #aliens.
Here, a program would recognize two hashtags. The hashtags are a marketing scheme that allows businesses and individuals to receive attention, sometimes virulently.
For instance, a year ago or so, Rhett and Link had a
hashtag war with Good Morning Maryland.