British slang for clear profit, good fortune or luck dates from the late 19th century when jam was considered a treat or luxury. Jam as preserve i.e. fruit and sugar jammed or compressed into jars dates from the early 18th century. To be in a jam is American and means to be in a predicament or trouble. It dates from the early 20th century and alludes to people being jammed together in tight, uncomfortable spaces, from which traffic jam is derived from c.1917, replacing the earlier traffic block, which dates from the late 19th century. Jam meaning to improvise musically, is African American from the late 1920s from which jam session is derived c. 1933, presumably alluding to producing sweet sounds.