doinchoisc

From to- +‎ in- +‎ com- +‎ seichid (“to declare”). Its original form was something like *do·inchosaig (compare in·coisig) which led to the t-preterite encountered in the poems of Blathmac due to the influence of saigid and aigid, which both had t-preterites.^[1]

The lenition instead of nasalisation of the combination of in- and com- is due to the prefix in- coming from both Proto-Celtic *en- and *eni- variants; *eni- caused lenition while *en- caused nasalisation.^[2] See tecosc for a nasalised doublet.

do·inchoisc (verbal noun tinchosc)

1. to inform, show
   • c. 815-840, “The Monastery of Tallaght”, in Edward J. Gwynn, Walter J. Purton, transl., Proceedings of the Royal Irish Academy, volume 29, Royal Irish Academy, published 1911-1912, paragraph 23, pages 115-179:

     Is maith immurgo lais do neuch do·inchoisc a llessai dóib cení·frecmairc coibsina.

     He thinks it well, however, for anyone, that one should show what is profitable to them, even though he does not ask for confessions.

2. to teach, instruct
   • c. 760 Blathmac mac Con Brettan, published in "A study of the lexicon of the poems of Blathmac Son of Cú Brettan" (2017; PhD thesis, National University of Ireland Maynooth), edited and with translations by Siobhán Barrett, stanza 87

     Tos·n-inchoisecht ind óen ré dénom nathrach n-umaide. Ba remib gabais crephel nathracha for díanteiched.

     At the same time he taught them the making of brazen serpents. Before them, terror seized the [other] serpents [and set them] upon swift fleeing.

• Gregory Toner, Sharon Arbuthnot, Máire Ní Mhaonaigh, Marie-Luise Theuerkauf, Dagmar Wodtko, editors (2019), “do-inchoisc”, in eDIL: Electronic Dictionary of the Irish Language