He used it in a short poem called "A Quiz" that appeared in the newspaper. The poem was a satirical commentary on the Irish political scene at the time.
While the exact meaning of "quiz" in the poem is a bit debated, it is generally understood to refer to someone who is easily fooled or tricked, or a person who is considered odd or eccentric.
The word quickly caught on and became popular in the English language, eventually evolving to mean a short test of knowledge.