In the Bible, there is a story about a boy named David and a philistine giant named Goliath. David conquers goliath with some stones and a sling. Stonily means in an unfriendly way, without showing any emotion. This joke is centered on the word "stonily" because we hear the word "stone" in it, which relates to how David conquered Goliath.
You ' ll never conquer me , Goliath said stonily .