Speaking forsoothly is a practice carried out in some parts of the SCA in which a more period atmosphere is attempted through means of speech. This is done through the use of Elizabethan English grammar, and the use of euphemism for overtly modern items.
Sometimes, however, this attempt results in using inappropriate terms which actually detract from the atmosphere (such as the use of dragon for a modern vehicle, farspeaker for telephone or halflings for children) and at other times it results in the use of incorrect psuedo-period phrases instead of perfectly good period ones (such as the use of remove instead of course in feasts or the use if smalls to refer to children). So speak forsoothly with care.
Despite the fact that Elizabethan English was only used for a short part of SCA period, its use to create a medieval atmosphere has its origins before the SCA. Until the early 20th century most translations of medieval texts were rendered into Elizabethan grammar.