Old English forsōth (see for, sooth).

• soothe from Old English:

  In Anglo-Saxon times to soothe was to show or prove that something is true. The first part of soothsayer (Middle English), ‘someone who can foresee the future’, is based on the same word and originally described someone who speaks the truth, while the archaic forsooth is simply another way of saying ‘in truth’. During the 16th century the meaning of soothe moved from ‘to corroborate a statement, back someone up in what they are saying’, to ‘humour or flatter someone by agreeing with them’. This finally led to the meaning ‘to calm, comfort, or placate’ which we are familiar with today.