The word "哨兵" is formed by combining the character "哨", which itself is composed of the mouth radical "口" indicating vocalization and the component "肖" suggesting resemblance or sound, originally meaning to whistle or to stand guard, with the character "兵", which depicts weapons and means soldier; together, they create a compound where "哨" modifies "兵" to specify a soldier who performs guard duties, thus meaning sentry.