means The amount added to the cost of something to set its selling price, or the set of tags and symbols that tell a document how to display its structure.

from A plain English compound of 'mark' and 'up' — 'mark' coming from Old English 'mearc,' meaning a boundary, sign, or token, and ultimately a Germanic root shared with words for borders and brands. To 'mark up' literally meant to write marks upon something. The pricing sense — annotating goods with a higher figure — is the older commercial meaning. The computing sense (as in HTML, HyperText Markup Language) borrowed the idea directly from publishing, where editors would 'mark up' a manuscript with handwritten instructions for the typesetter; the digital version simply swapped the editor's pencil for angle-bracketed tags.