Both are obsolete.
This Middle English word originally meant "an incorrigible, dogmatic old pedant," but eventually came to refer to an incorrect opinion that someone clung to. According to Kacirk, the word originated with an illiterate 15th century clergyman, who incorrectly copied the Latin word sumpsimus and read it in mass.
This Old English expression (probably borrowed from German) meant "fleeting weeks," and refers to what we today call a honeymoon. Flitterwochen is, obviously, a much better word.
16 Weird Forgotten English Words We Should Bring Back by Mental Floss--take a look, several other interesting words there.