Why do some Christians (Coptic orthodox and catholic) kiss the priest's hand? Considering kissing priest's hand is a christianity learning, where did this learning came from?  

Considering kissing priest's hand is a custom
Where did this custom came from, Why it is wide spreading ?
What is it's historical story ?
In Western Catholicism it is usually connected to the anointing of a newly ordained priest's hands with the Sacred Chrism. His hands have been set aside for God to do the Lord's work, and so some people (particularly in more traditional circles) kiss the hand. This is especially common with newly ordained priests, but also happens with priests in general sometimes.