This root is listed in Förstemann under VISC, though he doesn't provide a meaning and treats it dubiously.
Under the headword Wishart, the surname dictionaries by Reaney & Wilson and Hanks & Hodges list the root as VIZKR and VISKR respectively and give the meaning as "wise".
See also:
https://en.wiktionary.org/wiki/vizkrhttp://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:2003.02.0002:entry=vizkr