Infantry are soldiers who fight on foot — not from horses, and not mainly from inside tanks. A country's army is often split into parts; the infantry is the part that walks in, fights with its own weapons, and tries to take and hold ground. One soldier in that group is an infantryman (people now also say infantry soldier). Though actually, the word is a cousin of infant: it came through French, and from Italian or perhaps Spanish infantería, from infante, a foot soldier, originally a youth — a young fighter not high enough in rank to ride with the horse soldiers — and that youth-word goes back to Latin infans, “child.” So the baby word and the soldier word really do share a root. In English today, though, infantry never means children.