Matross was a soldier of artillery, who ranked next below a gunner

Gunner (rank)

Gunner is a rank equivalent to Private in the British Army Royal Artillery and the artillery corps of other Commonwealth armies. The next highest rank is usually Lance-Bombardier, although in the Royal Canadian Artillery it is Bombardier....

.

The duty of a matross was to assist the gunners in loading, firing and sponging the guns. They were provided with firelocks, and marched with the store-wagons, acting as guards. In the American

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 army a matross ranked as a private of artillery

Artillery

Originally applied to any group of infantry primarily armed with projectile weapons, artillery has over time become limited in meaning to refer only to those engines of war that operate by projection of munitions far beyond the range of effect of personal weapons...

.

The word is probably derived from French

French language

French is a Romance language spoken as a first language in France, the Romandy region in Switzerland, Wallonia and Brussels in Belgium, Monaco, the regions of Quebec and Acadia in Canada, and by various communities elsewhere. Second-language speakers of French are distributed throughout many parts...

 matelot, or from German

German language

German is a West Germanic language, related to and classified alongside English and Dutch. With an estimated 90 – 98 million native speakers, German is one of the world's major languages and is the most widely-spoken first language in the European Union....

Matrose, a sailor.