Oxen are used to pull the plows in the fields and the way they are kept in line is to use a goad- a long sharpened stick that sometimes had a metal point attached. When the ox would stray or pause, the plowman would prick the ox with the goad.
If the ox kicked back the plowman would have the goad ready and the ox would kick against the goad, or prick. The ox quickly learned the virtue of compliance with the owner's will. Acts 9:5 reads:
And he [Saul] said,
Who art thou, Lord?
And the Lord said,
I am Jesus whom thou persecutest:
it is hard for thee to kick against the pricks.
The ox kicking against the goad is what is referred to when Jesus was talking to Saul on the road to Damascas.
Additionally, Ecclesiastes 12:11 says
The words of the wise are as goads,...
Perhaps the words and testimonies of all those Christians who had tried to convince Paul that Jesus was the Messiah finally took effect.