A mark is a visible sign, line, or spot on a surface, or a symbol used for writing or grading. To mark means to make such a sign, to show where something is, or to give a score or label to something.
From Old English 'mearc' meaning 'sign, boundary, or trace', related to German 'Marke'. It has always been about making something noticeable or setting limits.
From 'question mark' to 'birthmark', the core idea is the same: something that stands out. Even 'mark my words' fits—you're asking someone to mentally put a highlight on what you’re about to say.
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