Really; factually • Often a question asked when one hears something astonishing or doubtful
[first form 1914+, second 1880+; the earlier form no kid is found by 1873]
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with no joke
Also,no laughing matter. A serious issue, as in Missing the last flight out was no joke, or This outbreak of flu is no laughing matter. The first term dates from about 1800, and the variant from the second half of the 1500s.