Green’s Dictionary of Slang

1. (US) to dress oneself up (for the purposes of illusion or fraud) [underpinned by SE dummy, a clothes mannequin].

1922

Wash. Times (DC) 21 Aug. 4/4: One day he dummied up in a lot of swell duds [...] and tooting solo about blue blood which chimed in with the monicker he pinched from the nobility, he started out to succeed on his nerve.

2. (US) to concoct a fraud, to fake something up.

1951		M. Spillane Long Wait (1954) 89: She was dummying the books and I took the rap for it.
1957		T.T. Chamales Never So Few (1958) 121: She ain’t dead. She’s dummying up.
1978		W. Diehl Sharky’s Machine 263: Tell them this Burns dummied up his own death.
1998		P. Doyle (con. late 1950s) Amaze Your Friends (2019) 25: Max had dummied up the guitar and accordion courses from his own collection of tutors [...] and old song books.