Jim-jam/Jim-jams

Jim-jam is what the OED likes to call “a fanciful reduplicated formation with vowel alternation”, like flim-flam, flip-flop etc. Jim-jam originates from the mid-16th century and meant a trivial knick-knack (which is yet another fanciful reduplication) but is now archaic. By the late 19th century, jim-jam had become slang for the heebie-jeebies (yet another fanciful reduplication). Jim-jams meaning pyjamas comes from a completely different source in that it derives from children’s nursery talk during the mid-20th century.