Cool: Cool is… well it’s cool. The usage of it to mean ‘fashionable’ or ‘good’ started in the early 1900s, but it’s etymology can be traced to way before that, in fact it has its root in the proto-Indo-Eurpoean culture.
The etymology of ‘cool’ in this sense, entwines with the etymology of the word ‘cool’ as in temperature. The proto-Indo-European (the earliest known common ancestor of all modern Indian and European languages) language had a word ‘gel’ which meant cold, this became ‘kala’ in Norse and then ‘kühl’ in German, well apparently the Germans were a pretty tough bunch, because they decided that what the Norse people meant by cold was really just a little below warm. The Germans also applied the term to a person’s temperament, as in ‘cool and level headed’, this might seem strange but temperature has been likened to many personality traits over the course of time, consider ‘hot headed’ and ‘cold shoulder’ for example. By the 16th century ‘cool’ was being used in English to describe people who were calm and level headed.
‘Cool’ became synonymous with ‘hip’ in the early 1900s, it joined English (in this sense) via the USA after being part of African American patois. By the 1920s it was firmly established as an idealism, that which people aspire to to; we have references in music fom the period including ‘Cool kind daddy blues’.