British slang meaning lucky or fortunate, or alternatively, covered with jam or sticky.
From 'jam,' possibly influenced by the idea that luck is 'sticky' or adheres to you. The British slang usage emerged in the 20th century, distinct from the literal jam-covered meaning.
British English loves turning food words into slang—'jammy' for lucky, 'cheesy' for fake, 'crumbly' for weak—it's a fun way their language plays with everyday objects to describe personality and luck.
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