According to Linguist/Historian EW Bullinger, this is an example of a figure of speech called paroemia. Paroemia is a wayside saying in common use.

 

"And whosoever shall not receive you, nor hear your words, when you depart out of that house or city, shake off the dust of your feet" (Matthew 10:14).

 

People who reject truth can do harm in two ways: by distracting others who may be ready to be called and by wasting the time of the person bringing the truth.

 

Christ told his disciples how to react to people rejecting their presence and their words. "Shake off" is from the Greek "ektinasso", the symbolic meaning of which is "express extreme contempt". The idea was to shake off (dust) for the cleansing of one's self, having been made dirty by the actions and words of those who reject the truth.