Origin: before 950; Middle English sothen to verify, Old English sōthian, equivalent to sōthsooth + -ian infinitive suffix; Modern English sense shift “to verify” > “to support (a person's statement)” > “to encourage” > “to calm”
O.E. soðian "show to be true," from soð "true" (see sooth). Sense of "quiet, comfort, mollify" is first recorded 1697, on notion of "to assuage one by asserting that what he says is true" (i.e. to be a yes-man), a sense attested from 1568.