Green’s Dictionary of Slang

1. (US) in a straightforward, open and honest manner.

1845		D. Corcoran Pickings from N.O. Picayune (1847) 73: I’m a butcher, right up and down, and I never followed no other business.
1854		‘Oliver Optic’ In Doors and Out (1876) 30: I told her, up and down, that she was not what she used to be [DA].

2. unrestrainedly; often as swear up and down.

1884		J. Harrison ‘Negro English’ in Anglia VII 263: To tell up an’ down = to tell plainly.
1886		E. Alton Among the Law-Makers 274: ‘I will recognise the higher authority of the Constitution!’ That is what [...] [the President] told [Congress], up and down! [DA].
1903		A. Kleberg Slang Fables from Afar 24: Pauderee called him a Vicious Cur and ripped him up and down the Back just awfully.
1951		Kerouac On the Road (The Orig. Scroll) (2007) 259: ‘What happened?’ I told him. He swore up and down.
1978		H. Selby Jr Requiem for a Dream (1987) 153: She just love us all up and down.
1995		J. Stahl Permanent Midnight 210: His girlfriend swears up and down he’s still fucked-up.
2005		J. Stahl I, Fatty 107: Sennett cursed me up and down.