'jam' (sense 'to press or squeeze') appears in early modern English; the precise origin is uncertain and likely dialectal, possibly from an earlier verb form like 'jam(m)en' used in regional speech meaning 'to press', conveying the idea of squeezing or crowding.

'jam' developed in English from informal/dialectal verbal forms (recorded variants such as 'jamen'/'jamman' in later Middle English and early modern usage) and stabilized in modern English as 'jam' with derived past 'jammed'.

Initially it mainly meant 'to press or squeeze'; over time the meaning broadened to include 'to block or become stuck' (mechanical or traffic contexts) and figurative senses such as 'to cram' or 'to cause congestion'.