'Ontological' traces to PIE *h1es- (to be) — the study of being itself, rooted in the verb 'is.'
Relating to the branch of philosophy that deals with the nature of being, existence, and reality; concerned with what kinds of things exist and the fundamental categories of being.
From 'ontology' + '-ical.' 'Ontology' was coined in the seventeenth century from Greek 'on' (ὄν, genitive 'ontos'), the present participle of 'einai' (to be), meaning 'that which is' or 'being,' combined with '-logia' (study of), from 'logos' (word, reason). The word was independently coined by the German philosopher Christian Wolff and the Swiss philosopher Jean-Baptiste du Hamel in the early 1600s to name the branch of metaphysics concerned with being as such. Key roots: on, ontos (ὄν, ὄντος) (Greek: "being, that which exists"), logos (λόγος) (Greek: "word, reason, study"), *h₁es- (Proto-Indo-European: "to be").
The PIE root *h₁es- (to be) behind 'ontological' is also the source of English 'is,' 'am,' 'are' (through Germanic), Latin 'esse' (to be, whence 'essence,' 'entity,' 'absent'), and Sanskrit 'asti' (is). The most abstract word in philosophy is built from the most basic verb in human language.