From the IARU's "Ethics and Operating Procedures for the Radio Amateur" (found at arrl.org):
SK: end of QSO: is used to end a QSO (SK = Stop Keying).
‘DIT DIT DIT DAH DIT DAH’ is the prosign ‘SK’ (from ‘stop keying’) and not ‘VA’ as published in some places (SK sent without inter letter spacing sounds the same as VA sent without inter letter spacing).
And from arrl.org's Ham Radio History: 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.
- Bruce K1FFX