You may be a 'boss'; you probably have a 'boss'. Unless, of course, you're one of those people who can say 'I'm my own boss'. 'Boss', meaning 'supervisor', 'overseer' or 'employer', was originally American slang but has been known here since early in the last century.
It was originally a Dutch word: baas. Baas meant, and means, 'master', though it may have its source in an earlier word for 'uncle'. In the 1620s, baas was used in American English to refer to the captains of Dutch ships, before going ashore and acquiring its anglicised spelling.
It was used as a replacement for 'master', which was never popular in the US, possibly because free white workers spurned it on account of its connection with slave labour. When 'boss' arrived here, it was used in workmen's slang and for humorous effect. 'Boss' can also be an adjective, meaning 'great'.
It was first recorded in the 1880s but is still heard in the US, according to the Urban Dictionary, and means 'supercool, fly, awesome to the max'.
It can also be used as a form of address. How's it going, boss?