By stealth, privately, without making an appearance. They spent their money in a hugger mugger way.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Hug * HugotontheonbiquiffinariansNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes