From Middle English step-, from Old English stēop- (“deprived of a relative, step-”, prefix), from Proto-West Germanic *steupa-, from Proto-Germanic *steupa- (“orphaned, step-”), from Proto-Indo-European *(s)tewp- (“to push, strike”).
Cognate with Scots step- (“step-”), West Frisian stiep- (“step-”), Dutch stief- (“step-”), Low German steef- (“step-”), German stief- (“step-”), Swedish styv- (“step-”), Icelandic stjúp- (“step-”). Related to Old English stīepan (“to deprive, bereave”). Not, however, related to the familiar English noun or verb step.
- A prefix used before father, mother, brother, sister, son, daughter, child, and so forth, to indicate that the person being identified is not a blood relative but is related through the marriage of a parent.
The short vowel is due to regular shortening of long vowels before consonant clusters in Early Middle English. All of the words that stēop- was prefixed to in Old English were consonant-initial, so shortening occurred exceptionlessly.