The hakama originated with the horse riding samurai of Japan. The split allowed the samurai to easily mount & ride horses while protecting their legs from brush and foliage.
During the continuation of this practice the samurai discovered that the hakama had the additional benefit of concealing the footwork of the samurai. This is especially beneficial during combat.
Over time symbolic meanings were assigned to the design of the hakama. One interpretation: The hakama has 7 pleats, 5 in front and 2 in back. Assigned to these pleats were the 7 virtues of Bushido:; Bravery (Yuuki), Benevolence (Jin), Etiquette (Rei), Justice (Gi), Loyalty (Chuugi), Honor (Meiyo), and Sincerity (Makoto). Bushido means “Way of the warrior”, and is a philosophy that was meant to enhance the samurai as they progressed through their lifework.
As the hakama is tied & knotted an 8th virtue is symbolized. This is known as Nen, and there is no direct translation of this meaning into English. Generally, Nen is a controlled and concentrated quality that is imbued by deep experience. During extensive training in a discipline a critical mass of knowledge/experience creates the virtue of nen, in Aikido we find this at the dan (black belt) level.
Finally, the himo of the hakama is knotted into a cross that is placed at the center of the body. This represents the Aiki-cross, and symbolizes the exoteric & esoteric, duality & oneness, humanity & divinity, material & spiritual, etc... Duality is represented by the vertical & horizontal aspects of the Aiki-cross, and oneness is represented by the center (Naka Ima (Center of Now)) of the Aiki-cross.