1. cause, means DN.ii.217, DN.ii.259; MN.iii.71 (samādhiṃ sa-upanisaṃ); SN.ii.30–SN.ii.32 (SN-a. = kāraṇa, paccaya); SN.v.25; AN.i.198; AN.iii.20, AN.iii.200 sq. AN.iii.360; AN.iv.99, AN.iv.336, AN.iv.351; AN.v.4 sq., AN.v.313 sq.; Snp.322 (= upanissaya Snp-a.331); Snp.p.140 (= kāraṇa, payojana Snp-a.503) Dhp.75 (cp. Dhp-a.ii.102 aññā nibbānagāminī paṭipadā).
2. likeness, counterfeit [= Sk. upaniṣad = aupamye Pāṇini i.4, 79] Ja.vi.470 (= paṭirūpaka C.).

if = Vedic upaniṣad, it would be fr. upa + ni + sad, but if, as is more likely, a contracted form of upanissaya, it would be fr. upa + ni + śri. The history of this word has yet to be written, cp. Kern,

Toevoegselen

s. v. & Divy.530 svopaniṣad