Source: Joe Catholic
If you went to a Catholic school, at least in the US in the 1960’s, you know that JMJ stands for “Jesus, Mary, and Joseph,” the names of the members of the Holy Family. And, there’s a better than average chance you were taught to write this at the top of your homework papers and tests.
The Venerable Bishop Fulton J. Sheen, host of the show Life Is Worth Living on the Dumont Network in the 1950’s and considered one of TV’s original televangelists, would write “JMJ” at the top of his blackboard before writing anything else on it.
“How to Think,” from Life Is Worth Living, 1955. Bishop Sheen writes “JMJ” at the top of his board around 1:45.
Since our parish, St. Ignatius, was staffed by the Jesuit Fathers, we didn’t put “JMJ” on our papers; instead, we wrote “AMDG,” Which stood for Ad Majorem Dei Gloriam (“To the Greater Glory of God”), the motto of the Society of Jesus. A classmate of mine claimed that it stood for “A Mouse Digs Gold”; I think he finished at a public school.
from The Sound of One Hand Typing