There were bosses in games ever since 1975, and I don't think I really have to define 'boss' here. But how did the word get stuck? And when was the first time when the challenging enemy was referred to as a 'boss'?
I find this immensely interesting, and have done some deep digging and made a video about the topic:
The earliest source I found is from a 1980 arcade operating manual of Space Panic, mentioning bosses. Another enemy in that game is a 'don'.
In early 1980's Japan, "bosu" (loanword from English) had a derogatory connotation to do with Mafia. Translating "bosu" would give you "boss", but in English the negative meaning is not present.
Old Kung Fu films use this term for mobsters too (Bruce Lee in The Big Boss).
From what I can find, the term's origin has been lost, however I did find some interesting details.
- The earliest usage seems to be from around 1983 in Joystick magazine, as discussed on Wikipedia.
Take a wild shot in Galaga-- you might fire (at) the Boss
- The term "Mini-Boss" appeared in the "Metroid" Instruction Booklet (referring to Kraid and Ridley, on page 15) when the game was released in 1986. Note that "Mother Brain" is not referred to as a "Boss" where she's discussed in the manual.
- "Boss" was spread more broadly by Nintendo Power, starting around 1988. I have a feeling that the term was coined inside of Nintendo as the magazine writers needed a term for the end-level baddies.
- The (translated) term "Boss" is also used in Japan. It might be that the term originated in Japan, but I have not found any information to back this up.
It could be that the word boss meant something different in a Japanese or something and since they didn't have a proper word for it in English, they just used the word boss. In the same way that Naruto says "Believe it!" in the anime Naruto, when in reality, it means something more like "Yeah!"
Another possibility, and don't quote me on this, is that a lot of the original bosses had minions or underlings working for them, think of king dedede and the waddle-dees/doos, Bowser and the koopas/goombas. When bosses first came out, no one questioned it, it just was so.
I will have to quote wikipedia on this one!
The first interactive game to feature a boss was dnd, a 1975 computer role-playing game for the PLATO system. One of the earliest dungeon crawls, dnd implemented many of the core concepts behind Dungeons & Dragons. The objective of the game is to retrieve an "Orb" from the bottommost dungeon. The orb is kept in a treasure room guarded by a high-level enemy named the Gold Dragon. Only by defeating the Dragon can the player claim the orb, complete the game, and be eligible to appear on the high score list.
The page in question is this one.
I hope that answers your question!