-si-

Originally from the plural stem of -nen.

-si-

1. Used in adverbs between a noun (usually referring to an article of clothing) and an external location case ending (adessive, allative, ablative) to refer to a state where one is (only) wearing that clothing.
2. Used in adverbs between a noun and an external location case ending with a more generic diminutive meaning.

-si- is always placed before the case ending, but it is basically arbitrary whether it attaches to the singular or plural stem of any given word. It appears that it is always attached to the strong stem for nouns that have gradation. Even further, in some cases it is preceded by an unetymological -i-, probably extracted by analogy from actually regular plural stems. The nominal form obtained as a result of attaching the locative case ending is considered to be in plural.

Only cases where the corresponding -nen derivative does not exist as an independent noun can be considered to use -si-, which has developed from the inflected forms of those derivatives by analogy, specifically.

-si-

1. (in verbs) reflexive affix which joins the prefix of a verb to the root

   atsitikti ― to happen

   pasiimti ― take (by self)

-si-

1. (with suffix -e) negative subjunctive marker

   ili nisile ― such that I would not eat

   • 18th century, Abdallah bin Ali bin Nasir, Al-Inkishafi‎^[1], translation from R. Allen (1946) “Inkishafi—a translation from the Swahili”, in African Studies, volume 5, number 4, →DOI, pages 243–249, stanza 11:

     هُنِلِزِ نَمِ كَلِ بَيْنِ ، لِوَاپُ نَسُرَ نِسِ كَتَيْ

     Hunelezi nami kali baini, liwapo na-sura nisi katae.

     Can't you tell it me plainly that I may admit it?

   1. negative imperative marker

      wasikisome ― they should not read it

2. negative relative marker

   mtoto asiyekuja ― the child who does not come

3. only used in -singe- and -singali-, negative marker for the conditional mood