Picture a shop where every box has a small paper label tied to it, and every box with the same label sits on the same shelf. A hashtag is that label, and the # is the string that ties it on. Write #NightMarket under your photo and the photo moves onto the night-market shelf, where a stranger can walk up and see it. From that one picture everything else follows: the label itself, the act of tying it on, and the piece of string on its own.
From earlier hash tag: hash (the # sign) + tag (a label). The # was proposed on Twitter in 2007 as a way of grouping messages, borrowed from the channel names used in IRC chat.
早期寫成兩個字 hash tag，也就是 hash（# 符號）加 tag（標籤）。2007 年有人提議在 Twitter 上用 # 把同一話題的訊息聚成一組，做法是從網路聊天室 IRC 的頻道名稱借來的。所以「符號＋標籤」這個組合本身就是它的意思。
Wiktionary etymology supplied in this packet (kaikki:3f6f69a190036ae4:1789733106:26062)
Two branches
標籤與貼標籤
紙牌本身，以及把紙牌掛上去的動作——名詞和動詞其實是同一個畫面的兩個角度。
noun · B1
A word or short phrase with a # sign in front of it and no spaces inside it, written in a post to say what the post is about. People tap it or search for it to find other posts on the same subject.
主題標籤
符號本身與口頭上的 hashtag
把注意力從整張紙牌移到綁牌的那條繩子——只講 # 這個符號；講話時把它唸出來，就等於當場替自己的生活掛一張牌。
noun · B2
The # sign itself. People also say the word 'hashtag' out loud in front of another word, usually as a joke, to make what they are saying sound like a line from a social media post.
井字號；口說的「hashtag」