Is it jam packed or jamb packed?

As a verb, jam means to squeeze tightly into a space, shove something into position, become stuck, or form a crowd. The noun jamb usually refers to a vertical piece on either side of a framed opening, as for a door, window, or fireplace.

Why do we say jammed packed?

In moderate to rapid informal speech, to avoid the “reset” necessary to pronounce the following p, the d following the collocation jammed packed would likely be elided, as in red and purple. The result would be jam packed, which may ultimately be the independent source for both Crocket and the American usage.