Green’s Dictionary of Slang

1. to baptize; cite 1906 (2) is a simply soaking.

1857		Ladies’ Repository (N.Y.) XVII 254: Now I am going to bapsouse you and make lambs of you.
1873		Columbia Herald (TN) 16 May 3/4: Nathanael was bap-soused in the waters near ‘Rickett’s Mills’.
1906		DN 3 125: There’s going to be a bapsouzin at the creek next Sunday.
1906		Wash. Times (DC) 6 Feb. 4/2: Lawd-a-mighty, look at dat. Yo am bapsoused fo’ sho’.
1926		E.M. Roberts Time of Man 119: ‘I baptize you...’ ‘Quit, I already been bapsoused.’.

2. in fig. use, to start off a new machine.

1958

Aberee V:I Apr. 1/1: We hope to have all this [i.e. setting up a new process camera] done in time to bapsouse it with this month’s magazine pages.