Matt 25:21

Reading Matthew 25:21, the story of the talents, I found the expression
"eiselthe eis tEn kharan tou kuriou sou."  ("Enter thou into the joy of
thy lord," said the KJV; "Come and share your master's happiness!" chirps
the NIV.) 

Was this a conventional expression at the time, among Greek or
Aramaic speakers?  Does it have a history of mystical interpretation, as
it seems to invite?