Definition of stitch up:
(v) :
To join or close by sewing.
(v) :
To fabricate (e.g. a legal case).
(v) :
(British, Australia, slang) To maliciously or dishonestly incriminate someone; to set up (in the sense trap or ensnare)
(v) :
(Australia, slang) To prank, trick, or deceive (a person), whether or not malice is intended.
(v) :
(Australia, slang) To complete arrangements for (a situation), especially clandestinely or prematurely and for one's own benefit.