The story
From throb + -er. In sense 1, after version 1.0 of the Netscape web browser, which used an animation of its logo (a big blue N) expanding and contracting, as in a throb.
Etymology from Wiktionary, CC BY-SA 4.0.
throbber
throb·berToday it means: A widget consisting of a static image that animates during execution of a task. Full definition →