Fend And Prove Meaning

(idiomatic) To engage in argument.

Example: 1690, John Locke, An Essay Concerning Human Understanding
  The dexterous management of terms, and being able to fend and prove with them, I know has, and does pass in the world for a great part of learning; but it is learning distinct from knowledge; [....]
1818, John Dunton, The Life and Errors of John Dunton, Citizen of London
  All Pulpit-fools are enemies to Love, If e'er they think, 't is how to fend and prove.