Question about specific sentence. [Spoiler]

SPOILER AHEAD SPOILER AHEAD SPOILER AHEAD


Hi all,

I wanted to ask, at a certain point, Ladd's character is talking to Lake's character and he says: "I get along very well with Paul because he's on the dead up-and-up"


What does that mean? It must be old fashioned lingo, can anybody help me out?

I want to know specifically in relation to the story, and the general meaning.

Thanks!