1930-35; perhaps jam1, or by shortening of jamboree
Examples from the web for jam session
In the informal situation, you can find yourself in a jam session, where anything can happen.
Tonight travelers and locals alike gather around platters of barbecued chicken for a jam session.
If you're lucky, you might see a big-name professional artist pop in for a jam session.
Second, each musician is directly connected with the other musicians in a jam session, instead of being routed through a server.
The material isn't organized in any formal way but works as a mosaic that has the feel of a jam session.
Afterwards, individuals are invited to stay around and participate in a jam session.
Tennis enthusiasts are invited to participate in our jam session.
British Dictionary definitions for jam session
jam session
noun
1.
(slang) an unrehearsed or improvised jazz or rock performance
Word Origin
C20: probably from jam1
Slang definitions & phrases for jam session
jam session
noun phrase
Agathering of musicians, esp jazz musicians, playing freely and for one another, but sometimes in a public performance; jam: Here's Dodo on the piano and Tiny on the bass. Looks like we'll have a fine jam session(1930s+ Jazz musicians)