Etymology Dictionary

JAM (1),  to press, squeeze tight.  (Scand.)   'Jam, to squeeze;' Halliwell.   'Jammed in between the rocks;' Swinburne, Travels through Spain (1779), let. 3, p. 8.   'Jam, to render firm by treading, as cattle do land they are foddered on;' Marshall's Rural Economy of Norfolk (E. D. S. Gloss. B. 3).   The same word as cham, or champ.   'Cham, to chew or champ;' Palsgrave.   'Champ [with excrescent p], to tread heavily, Warwickshire; to bite or chew, Suffolk;' Halliwell.   Whence also:  'Champ, hard, firm, Sussex;' id.; i.e. chammed or jammed down, as if by being trodden on.   See Champ, which is of Scand. origin.   ¶ For the common and regular change from ch to j, see Jaw, Jowl.

JAM (2),  a conserve of fruit boiled with sugar.  (Scand.?)   In Johnson's Dict.   Of uncertain origin, but most likely from Jam (1).   The following quotation suggests that it may mean a soft substance, resembling what has been chewed.   'And if we have anye stronger meate, it must be chammed afore by the nurse, and so put into the babe's mouthe;' Sir T. More, Works, p. 241 h.   See Champ.