(from Late Latin, 1526, genuflectionem (genuflexio), from stem of genuflectere "genuflect", from Latin genu, "knee" + flectere< "to bend")

The practice of genuflecting has no claim to origin from ancient times

Genuflection appears to have been introduced and gradually to have spread in the West during the later Middle Ages, and scarcely to have been generally looked upon as obligatory before the end of the fifteenth century.

The older Roman Catholic Missals make no mention of it. One Catholic authority gives A. D. 1502 as the date of the formal and semi-official recognition of these genuflexions.

Even after it became usual to raise the consecrated Host and Chalice for the adoration of the Faithful after the Consecration, it was long before the priest's preceding and following genuflexions were insisted upon.

The genuflexions now indicated at such words as "Et incarnatus est", "Et Verbum caro factum est", etc, are likewise of comparatively recent introductions, though in some cases they replace a prostration that was usual, in ancient times, when the same sacred words were solemnly uttered.

The Carthusian custom of bending the knee, yet so as not to touch the ground, is curious; and has interest from the historical point of view as testifying to the reluctance formerly felt by many to the modern practice of genuflecting.

Women, as well as men, must genuflect before the Blessed Sacrament

The simple bending of the knee, unlike prostration, cannot be traced to sources outside Christian worship; therefore, the pagan and classical gesture of adoration consisted of standing before the being or thing to be worshipped, in putting the right hand to the mouth (ad ora), and in turning the body to the right.

The act of falling down, or prostration, was introduced in Rome when the Cæsars brought from the East the Oriental custom of worshipping the emperors in this manner as gods.

The Roman Catholic liturgical rules for genuflecting are now very definite.

—Excerpts from The Catholic Encyclopedia,
Volume VI; Online Edition.

A unit of other flect-, flex- words.