Until recently, I’d assumed that lunch was a clipping of luncheon. Come to find out, the words originated separately.
According to the OED, luncheon is of uncertain origin, but lunch is thought to have come from Spanish lonja, “a slice.” Both luncheon and lunch first referred to a thick piece of something, such as “a hunk of bread.” In time, both words came to mean “a light repast between mealtimes.”
On the Ngram Viewer, lunch and luncheon cruise along in tandem until about 1880, when lunch pulls ahead. Luncheon peaks in the 1920s and then declines. By then, some middle class speakers had come to believe that lunch was a slang word derived from luncheon and complained about its use by the younger generation.
Both words still refer to a meal lighter than dinner, but now have markedly different connotations.
A Google search shows 625,000,000 results for lunch compared to 43,200,000 for luncheon. The latter has acquired connotations of formality and a kind of prissiness that do not attach to lunch.
One “has lunch” but “attends a luncheon.” A Web search suggests a certain gender bias: “ladies’ luncheon” brings 374,000 results compared to 34,900 for “men’s luncheon.”
The word lunch has found its way into several idioms.
out to lunch
A person who is “out to lunch” is unaware of what is going on around him:
One [presidential] candidate is simply out to lunch, unaware until recently that there was even such a thing as Common Core.
to lose one’s lunch
“To lose one’s lunch” is “to throw up or vomit”:
Once the sight would have made me lose my lunch, but it didn’t anymore. Was it a bad sign that I didn’t throw up on the corpses anymore?
to eat your lunch
To have someone “eat your lunch” is “to be soundly defeated in some competition”:
If you don’t develop a marketing strategy, your competitors will eat your lunch.
no such thing as a free lunch
All services, gifts, and ostensibly generous gestures will have hidden costs that the recipient must pay in the end:
On the Internet, as elsewhere in life, there is no such thing as a free lunch. [Free or low-cost online services need to be paid for somehow.]
A “liquid lunch” is a “midday meal that leads to drunkenness.”
An alarming 76 per cent of employees are coming back to work drunk after taking a “liquid lunch,” according to a new survey.