Ton.

With irregular t- (as in tausend) and widespread dialectal -ā- → -ō- from Middle High German dāhen, tāhen, inflected form of dāhe, tāhe, from Old High German thāha, dāha, tāha, from Proto-Germanic *þanhǭ, from Proto-Indo-European *tenk- (“to thicken, be solid”), related to *þinhtaz (“dense, tight”). Cognate with Old English þō, Old Norse þá, Gothic 𐌸𐌰𐌷𐍉 (þāhō). The oldest spelling is Toh(e)n, which then became Thon as part of a general habit of writing -th- in the vicinity of long vowels. During the spelling reforms of the early 20th century, all th-spellings in inherited words were reduced to -t-, thereby making Ton one of the very few words in which Proto-Germanic *-h- is not reflected (compare the same in Träne).

From Middle High German tōn, from Latin tonus, from Ancient Greek τόνος (tónos).