I’m not native English speaker, so I wonder why forces like policemen and firemen and such use Mayday instead of the simpler Help. What is origin of this habit?
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Mayday comes from the french venez m'aider, which translates to "come help me". This was simply formalized as official in the 1940s. Any other phrase could have been used; mayday is just what stuck.
Technical terms like "mayday" have several advantages over just use an already existing common term like "help!":
It may have a slightly different definition than any existing common word. This isn't particularly true for "mayday", although I think using "mayday" instead of "help" indicates someone is a member of whatever field and therefore can be treated as such during the rescue.
A common term like "help" can be used in all sorts of situations, so someone saying help on the radio might cause confusion; they might not be calling for help, there are lots of situations someone would just say the word. Yelling "mayday mayday mayday" on a radio isn't ambiguous; everyone immediately knows that this is a call for help.
Specialists like to be cool by having their own languages.
I think this explains everything. I quote here the corresponding paragraph of the article in Wikipedia:
The Mayday callsign was originated in 1923 by Frederick Stanley Mockford (1897–1962). A senior radio officer at Croydon Airport in London, Mockford was asked to think of a word that would indicate distress and would easily be understood by all pilots and ground staff in an emergency. Since much of the traffic at the time was between Croydon and Le Bourget Airport in Paris, he proposed the word "Mayday" from the French m’aider. "Venez m'aider" means "come help me."