sooth: [OE] Sooth ‘truth’ (which now survives in current usage only in the compound soothsayer [14]) goes back ultimately to Indo-European *sntyós (possible ancestor also of English sin). This was a derivative of the base *es- ‘be’, and hence etymologically means ‘that which is’. It passed into prehistoric Germanic as the adjective *santhaz.
As in English, in most other Germanic languages the word has now died out, but it survives in Swedish (sann) and Danish (sand) as an adjective meaning ‘true’. From the Old English form sōth a verb was formed, sōthian ‘prove to be true’, which has evolved into modern English soothe. Its present-day meaning did not emerge, via intermediate ‘confirm’ and ‘please or flatter by confirming or agreeing’, until the 17th century. => soothe[sooth etymology, sooth origin, 英语词源]