Supporting the communication, language, and literacy development of children with complex communication needs: State of the science and future research priorities J Light, D McNaughton Assistive Technology 24 (1), 34-44, 2012
Cool beans is a curious phrase that uses nonsense to bring humor into a joyful exclamation. It originated in American pop culture during the late 1960s. Slang of this time is known to center around illogical phrasings of words, and cool beans is among the most durable lengthy late 60's era terms. It is used when referring to something with a positive connotation, for example, That party was cool beans..
That 'let out' meaning was probably influenced by an earlier meaning of 'spill', that is, 'kill' and the subsequent usage 'spill blood', which was in common use by the 14th century. The earliest uses of 'spill the beans' come from the USA. The meaning of the phrase was then something like 'spoil the beans' or 'upset the applecart', which harks back to the supposed Greek knocking over of a bean container. The first example I can find is from The Stevens Point Journal, June 1908: