Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
guess (v.)
in locutions such as I guess I’ll go now and She guessed she didn’t want to see him, are Standard English, although probably not often employed in the most Formal or Oratorical contexts. Such uses were brought from Britain and kept alive by the new Americans, while atrophying and dying at home. Only recently is I guess now once again being used a bit in Britain, presumably borrowed back from the United States.
