[Creative_Commons]

Miu_Miu писал (а)

you felt like my brains start to boil ….


Also I remember my reaction when I received something like : please send me this doc by Tuesday COB .
Later I understood that this is close of business which means end of business day .

This isn't something you actually learn at school or university, because the majority of teachers have no idea of all these abbreviations, and how and when to use them. You can only learn something like that when you run into it working with native speakers or in the English-medium business environment.