1 [with object and adverbial] squeeze or pack tightly into a specified space:four of us were jammed in one compartmentpeople jammed their belongings into cars [no object, with adverbial]:mum, dad, and I jammed into the pickup truck
push (something) roughly and forcibly into position or a space:he jammed his hat on
[with object] crowd on to (a road or area) so as to block it:the streets were jammed with tourist coaches
[with object] cause (telephone lines) to be continuously engaged with a large number of calls:listeners jammed a radio station’s switchboard with calls
2become or make unable to move or work due to a part seizing up or becoming stuck: [no object]:the photocopier jammed [with object]:the doors were jammed open
[with object] make (a broadcast or other electronic signal) unintelligible by causing interference:they were jamming broadcasts by the pirate radio ships
3 [no object] informal improvise with other musicians, especially in jazz or blues:he had the opportunity to jam with Atlanta blues musicians
1an instance of a thing seizing or becoming stuck:paper jams
short for traffic jam.she was held up in a jam on the M25
Climbing a hold obtained by jamming a part of the body such as a hand or foot into a crack in the rock.
2 informal an awkward situation or predicament:I’m in a jam
3 (also jam session) an improvised performance by a group of musicians, especially in jazz or blues.
jam on the brakes
operate the brakes of a vehicle suddenly and forcibly, typically in response to an emergency.
early 18th century: probably symbolic; compare with jag1 and cram