This is an alternative form of shi- which comes about in some words as a result of coronal harmony. See, for example, sikʼis, sitsilí. This once firm rule has become optional, and today most Navajo speakers actually prefer to ignore the rules of sibilant (coronal) harmony, especially when there is relatively greater distance between the 1st-person shi- and the alveolar sibilant in the root.
si- (basic noun prefix)
- Basic (augmentless) form of isi-
si- (subject concord)
-si- (object concord)