Þagsi

As defined by the Cleasby & Vigfusson Old Norse to English dictionary:

þagsi

adj.; the parent-word of the mod. Dan. tavs (silent) has hitherto not been found in the old Icel. literature; it should be þagsi, but most if not all words of that form were obsolete as early as the 12th and 13th centuries, and have disappeared in mod. Icel., as hugsi, staðsi, heitsi …; we believe it is preserved in the corrupted ‘þegn varð’ in the transcript of the Íb., where we propose to read, svá at allir menn myndi ‘þagsi verða’ (= Dan. blive tavse) meðan hann mælti at lögbergi, so that all men would become silent whilst he spoke on the Law-bill, Íb. 7. In an ancient vellum a , with the upper end of the s faint or blotted out, and an n or  would be distinguished with difficulty; and as to an Icel. transcriber þagsi was quite an unknown word, while ‘þegu,’ ‘þagu’ were familiar syllables, he would choose the latter; the vellum itself was lost soon after the copy had been taken in A. D. 1651. In Rb. (Ísl. i. 385) the word has been paraphrased into ‘þegja;’ see the Academy, vol. i, p. 278.

Possible runic inscription in Younger Futhark:ᚦᛅᚴᛋᛁ
Younger Futhark runes were used from 8th to 12th centuries in Scandinavia and their overseas settlements