From the 1969 ARRL "The Radio Amateur's Operating Manual"

Many of the expressions and procedure signals still in use in
radiotelegraph had their origins in the early days of the landline 
telegraph - long before Marconi sent his letter "S" across the 
Atlantic.  In sending formal messages by CW, the first thing a 
beginner hears is "don't send punctuation. Separate the parts of 
the address from each other with the prosign AA."  This is ironic, 
because in the American Morse Code the sound didahdidah is a comma 
and was doubtless the origin of our prosign.  Originally, a correctly 
addressed letter was punctuated with commas following the name and 
the street address, each of which was (and still is) on a separate 
line although the commas have been dropped, even in mail addresses on 
letters.  The comma was transmitted by Morse operators and thus, AA 
came to mean that the receiving operator should "drop down one line" 
when sent after each part of the address and it is so defined in the 
operating manuals of the time.  Our familiar prosign SK also had its
origin in landline Morse.  In the Western Union company's "92 code" 
used even before the American Civil War, the number 30 meant 
"the end. No more".  It also meant "good night".  It so happens that 
in Landline Morse, 30 is sent didididahdit daaah, the zero being a 
long dash.  Run the 30 together and it has the same sound as SK. 

         -Louise Ramsey Moreau W3WRE/WB6BBO


   Where Did They Come From?
The end-of-message signal, AR comes from the American Morse letters
FN, meaning 'finish'.   SK, from the American Morse 30, meaning
half-past the hour, the end of an operator's shift.   ES, for 'and'
from the American Morse symbol for '&', used extensively in written
English in earlier times.  And when old-timers send a long dash for
'zero', they are actually sending the correct American Morse symbol.
   History casts a long shadow.


   Procedural Signals (Prosigns) for Morse Code

C Q - Calling any station (does any ham *not* know this one?) 
AR - over, end of message 
K - go, invite any station to transmit 
KN - 'X' go only, invite a specific station to transmit 
B K - invite receiving station to transmit 
R - all received OK 
AS - please stand by 
SK - end of contact (sent before call) 
C L - going off the air (clear)