It looks like it first shows up in books in around mid 1950s
https://books.google.com/ngrams/graph?content=meritocracy&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cmeritocracy%3B%2Cc0#t1%3B%2Cmeritocracy%3B%2Cc0
