Green’s Dictionary of Slang

to humbug.

1857		‘Ducange Anglicus’ Vulgar Tongue (1859) 22: Soaped him over, Humbugged him.
1880		Cornishmen 4 Mar. 4/5: His audience was of a different sort to those he tried to soap over at Helston.
1886		S. Watson Wops the Waif 4/2: Who’s yer tryin’ to soap over, old Sholomon Soft-soap?

(N.Z.) to get started, to set off.

1946		H.P. Simpson If You’d Care to Know 100: An expression in common use Down Under is soap the geyser [...] It is confined almost exclusively to New Zealand [...] In the Thermal Regions there is a particular geyser that could be made to perform by throwing soap into it, and it was frequently soaped so that it would put up a display for the delight of important visitors. So today, when a New Zealander says, ‘Come on, let’s soap the geyser,’ he means ‘Come on, let’s get going’, or ‘Let’s start’ [DNZE].
2003		McGill Reed Dict. of N.Z. Sl.

(US) to act kindly toward, to ingratiate oneself with; thus soaping up n.

1893–4		H. Lawson ‘Stiffner and Jim’ in Roderick (1972) 127: Then, when I thought I’d licked him into form, I cooled down and soaped him up a bit.
1953		F. Paley Rumble on the Docks (1955) 315: Why all this soaping up when they didn’t have to do it?

see go soak your head! under soak v.¹