The only answers I can find talk about right-hand dominance or say the phrase comes from the bible (the phrase being in the bible doesn't make it the origin and does nothing to explain it's meaning) I'm curious if there is a known origin for this phrase. Is there a specific fable or event that explains why someone on your right-hand side is to be so trustworthy?
I would argue that the prevalence of the "right hand" in the Bible is the origin as it is the reference point for so much Western literature. I think the word "right" may not purely be a reference to direction but also right as in right vs. wrong. The first appearance, biblically, seems pretty significant. In Genesis, when land is partitioned to Joseph's sons, Ephraim was ceded Israel, which was on Joseph's "right hand."
An in-depth look at the symbolism of right vs. left in Biblical times (not just the Bible specifically) can be found here: http://www.jewishvirtuallibrary.org/jsource/judaica/ejud_0002_0017_0_16755.html