Student or Learner
in what sense "best man" (the man who helps the bridegroom at a wedding ceremony) is better than the bridegroom himself? I mean, maybe you know the origin of this word (best man)? And is it common to use this combination of words in other, not idiomatic sense (eg. he's the best man in the platoon (meaning "the best soldier"); he's the best man for this job ("the best specialist") etc.).
Well, it's not the question of being better than the bridegroom himself. Rather, the best man is better than other men who could be theoretically eligible. The groom extends this honor to someone who is close to him, generally either a brother or his closest male friend.
Take Our Word For It, page two, Words to the Wise
From D.K. Shaw:I'm curious as to the origins of the term best man. Is it simply the "best friend" of the groom, or is there more to it? I recently ran across an unsubstantiated theory that it referred to the "best swordsman" in the clan who would help fend off the bride's family if they came to steal her back [having been kidnapped for marriage'. I imagine the truth is much more prosaic.Well, you mention clan, and best man does, in fact, come from Scotland. However, it simply meant "a friend of the groom". There was also a best maid who was equivalent to our maid of honor. Best man dates from 1814 in English.
The practice of marriage by bride abduction is certainly performed in some cultures (such as the Hmong of Laos) and remnants survive elsewhere (Tibet, for example). While we have heard the story that the best man was a friend who helped the groom capture his bride from another tribe or clan, we've yet to see any evidence of this or even that bride-abduction was ever practiced in Britain.