Wiktionary
peg down
vb. 1 To fasten something to the ground using pegs. 2 (context idiomatic English) To identify clearly.
WordNet
Usage examples of "peg down".
Then he drove the long peg down with a wooden mallet which the negro handed to him.
It sought and dug and hunted, sniffing through the dark tunnels of its mind, and there was no beginning it could peg down tight and solid.
So it may be that Parleyvoo is nervous about strangers they can't peg down.