Possibly from the same root as patior (“to suffer”); this may be Proto-Indo-European *peh₂-(i)- (“to hurt”) (this is supported by Pokorny), but its standard reconstruction as *peh₁-(i)- (or *peh₁y-) creates phonetic problems for the Latin. Neri (2007), following Vine, derives it Proto-Indo-European *p(e)-ai-ni- (“not entirely; from whom has been taken away; who takes away”), from *pe- (“away”) + *h₁ai- (“to take, give”), but this is doubtful since a PIE *h₁ai- sequence is phonologically invalid and the preverb *pe- (“away”) is itself uncertain. So, the etymology is unresolved. The original meaning was likely “lacking, missing” (> “falling short”), as preserved in the related words paeniteō (“to cause dissatisfaction/regret”) and paenūria (“shortage”).