Think of a small machine that goes tick … tick … tick, one short beat for every single thing that happens. Each beat leaves a mark behind: a price printed on a paper strip, a line sliding across a screen, a beat inside your chest, a check drawn beside a name on a list. The word ticker points at whatever is doing that steady, one-at-a-time marking — never at a finished, still picture of everything at once.
From the word’s history
The packet's source entry says the word is simply tick + -er: the thing (or person) that ticks. English tick covers both the small repeated sound and the small written mark, and both of those roads are used by this word.
資料上寫 ticker 就是 tick 加上 -er：做 tick 這件事的東西或人。英文 tick 同時有「滴答聲」和「打勾」兩個意思，這個字兩條路都走了。
Wiktionary entry for ticker, supplied in this packet: "From tick + -er."
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