A verb infamous for its complicated origin. There are two major theories on its origin:[1]
A modern theory derives the verb from a suppletive conflation of a nasal-infix present *lungeti from Proto-Indo-European*h₂lewgʰ-(“to announce”) and a preterite derived from *tek-(“to reach out, receive with one's hands”). The initial t- outside the present would thus come from analogy with the preterite. This theory dismisses a connection with Gaulishtoncsiiontio.[2]
An older, more traditional theory supposed descent from Proto-Indo-European*th₂négti(“to touch, grasp; to take”), ne-infixed verb from the root *teh₂g-(“to touch; to take”), with semantic divergence (i.e. to “take (an oath)”); compare Latintangō(“(I) touch”), especially in the figurative meanings “affect; enchant”, Proto-Germanic*þakkōną(“to touch lightly”) and Ancient Greekτεταγών(tetagṓn, “having seized”, ptcp.). The o-grade in Old Irish however would be unexplained.
^ Schumacher, Stefan, Schulze-Thulin, Britta (2004) “Urkelt. *tu-n-g-e/o- ← 2. *lu-n-g-e/o- ‘schwören’”, in Die keltischen Primärverben: ein vergleichendes, etymologisches und morphologisches Lexikon [The Celtic Primary Verbs: A comparative, etymological and morphological lexicon] (Innsbrucker Beiträge zur Sprachwissenschaft; 110) (in German), Innsbruck: Institut für Sprachen und Literaturen der Universität Innsbruck, →ISBN, pages 648-52
^ Gordon, Randall Clark (2012) “to-n-g-”, in Derivational Morphology of the Early Irish Verbal Noun, Los Angeles: University of California, pages 317-319
