mit jemandem Pferde stehlen können
to be able to rely on someone completely; a trustworthy partner in crime
Origin
Attested since the early 17th century (e.g. in Zacharias Theobald, 1621). Because horse theft was severely punished in earlier times, horse thieves had to be able to rely on each other completely - hence the phrase stands for unconditional trust.
Building blocks
Words in this expression
Examples
Auf Anna ist Verlass — mit ihr kann Pferde .
You can always count on Anna — she's someone you can go through thick and thin with.