Many baby boomers remember the wildly popular JAMS shorts of the 1960s and 1980s. JAMS were closely associated with the California and Hawaii surf scenes. The JAMS brand still exists, and has grown into today's Jams World clothing and accessories line. _Wikipedia
Growing up in southern California among surfers I know this word as a generic term spelled without capital letters. It was used for any baggy knee length swim shorts with drawstring waist that came in loud floral patterns. Jams is short for pajamas and the comfort of pajamas is implied.