This is the 7755th most frequent French word.
sainz
"Sainz" is not a French word. It may be a misspelling or a proper noun (e.g., a surname).
In this sentence, 'sainz' serves to mean 'without', indicating 'without reason'.
Je vois qu'il agit sainz raison.
I see that he acts without reason.
Here, 'sainz' is used to imply 'without', as in 'without foundation'.
Les conseils sainz fondement ne m'intéressent pas.
Advice without foundation does not interest me.
In this example, 'sainz' denotes 'without', referring to 'without flaws'.