stief-:

From Middle Dutch stief-, stiep-, from Proto-Germanic *steupa- ?(“bereft, pushed out”). Cognate with German stief-, English step-. The form with -f- arose through assimilation before the word vader ?(“father”); the generalization of this form may or may not have been reinforced by High German influence (where the -f- is regular). Compare the same in Middle Low German stēf-.