The concept of 'being cool' was invented by American teenagers in the 1950's. Prior to this the term 'Cool' had been pejorative, used to indicate insufficient enthusiasm.
The coolest person ever was Bob Dylan, during the years 1965 and 1966. Unfortunately, the strain of being so cool for so long caused him to crash his motorbike (a Triumph Bonneville, naturally) and break his neck. Although he recovered physically, he was never truly cool again. (And his music went downhill as well.)
Being cool is actually very easy. All that is required is
1. Pretend to have an IQ of approximately 90 (i.e. not stupid, but definitely 'slow'.)
2. Wear sunglasses.
Various other items of clothing, attitudes, etc, are or have once been considered to be 'cool'. These are however merely transient fashions and as such are liable to become suddenly 'uncool' at a moment's notice, thereby putting the would-be 'cool' person into an embarrassing position. The simplest and safest way of 'Being cool' is therefore to stick to the two essentials above.
Note that in the mid 1990's, various computer enthusiasts began to describe bits of software, and occasionally hardware, which they considered to be interesting as 'cool'. This usage has to be stamped out. Computers may be many things (useful, irritating, irrational, etc) but they will never be cool.