The ""step"" prefix......

[Quasimodem]

… in “step-mother/father”. Step what? Down?

Thanks

Quasi

[Derleth]

From http://www.m-w.com/

One entry found for step-.

                 Main Entry: step-
                 Function: combining form
                 Etymology: Middle English, from Old English stEop-; akin to Old High
                 German stiof- step-, Old English astEpan to deprive, bereave
                 : related by virtue of a remarriage (as of a parent) and not by blood
                 <stepparent> <stepsister>

So, apparently, a stepfather is a father for those deprived of their biological fathers.

[hajario]

According to Webster it origninates from the Old English astepan which meant to deprive or bestepan which meant to bereave.

Haj