How did policemen come to be called 'cops?'

The term originated in the United States back in the 1930s. The "beat cop", or the officer who patroled the streets on foot, uniform had copper buttons on it. Street thugs and gangs at this time started calling police officers "coppers". Hollywood later immortalized this term in movies. Remember the line "You'll never take me alive, Coppers!" Later, the term has been shortened to "cops" and has remained as our current eumphanism for police officers today.

May be from US.

maybe from the word caps because they are maiuscol :)