It comes from the verb "depend", which itself has roots in Old French and Latin:
* Old French: *dependre* (to hang down, to be suspended)
* Latin: *dependere* (to hang down, to be suspended)
"Dependability" was formed by adding the suffix "-ability", which indicates the quality or state of being able to do something. This suffix was added to the word "depend" in the 19th century, likely stemming from the need for a noun to describe the characteristic of being reliable and trustworthy.
So, in essence, "dependability" literally means the state of being able to "hang down" or be "suspended" upon, highlighting the idea of being reliable and trustworthy enough to lean on or rely on.