A pronoun for the “same thing”, also usable as an impersonal reflexive pronoun meaning “same, self-same”, with a variant form imba (VT47/37-38). It is a combination of im- “same” and ma “thing”.
Conceptual Development: Demonstrative, Relative, and Correlative Stems (DRC) from 1948 had ᴹQ. ona “that identical fact” based on ᴹQ. on- “same”, usable only of abstract facts and notions rather than functioning as a true neutral pronoun; originally it was {alapa “the same thing”} (PE23/104 and note #56). Elsewhere in the document Tolkien initially said these forms could also be adjectives, before deciding the proper adjective form was ᴹQ. onya (PE23/104 and note #70).
imma *"itself", impersonal reflexive pronoun referring to the "same thing" (VT47:37) as the subject; compare immo.