Small arachnid parasites that feed on blood, or the plural of tick referring to checkmarks or small clicking sounds.
Middle English 'tike', possibly from Old English 'ticia', related to Middle Dutch 'teke'. The checkmark sense comes from the quick stroke motion, while the sound sense is onomatopoetic, imitating the actual ticking noise.
Ticks are remarkable for their patience - they can wait years for a host, then detect approaching animals through carbon dioxide, vibrations, and heat. The word's multiple meanings all share the concept of small, persistent actions: parasites that stick, marks that check, sounds that repeat.
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