According to Acharya Chanakya these are three things which should never be touched with foot
Acharya Chanakya says
Anal Vipr guru Dhenu puni, Kanya kuwari det
Balak ke aru vridh ke, Pag na Lagavahu Yet
1)fire is considered sacred to the god. That's why touching fire with foot is considered unlucky
2)Similarly, master, Brahmins and the sacred cow are also considered sacred. Touching any one of these with foot is believed to insult them.
3)Touching these seven things( fire, the spiritual master or a brahmana, a cow, a virgin, an old person or a child.) with foot even unknowingly is considered inauspicious