means A small blood-sucking arachnid that latches onto skin to feed, notorious for spreading disease (the word also covers a light clicking sound, a checkmark, or a moment of time, but the bug is the biter here).

from From Old English 'ticia,' the name for the parasite, with cousins across the Germanic languages — Dutch 'teek' and German 'Zecke' all point back to a shared ancestor for the little clinger. The other 'ticks' arrived separately: the clock's 'tick' and the checkmark 'tick' are imitative coinages, born from the small clicking noise itself, while 'in a tick' borrows that same beat of time. So one spelling quietly holds a bug, a sound, and a mark — only the first one bites.