shareholder

Formed in English simply by joining share and holder. Share traces back through Old English sċearu to Proto-Germanic *skarō and ultimately Proto-Indo-European *(s)koréh₂, all carrying the sense of a division or portion cut from a whole.

Holder itself is built from hold (Old English healdan, from Proto-Germanic *haldaną) plus the agent suffix -er, which came into English through Old English -ere from Latin -ārius. Put together, shareholder simply names the person who holds a portion of something.

The -er in holder is the same ending found in countless English agent nouns, having passed from Latin -ārius through Old English -ere into Middle English -ere and finally English -er.