The name derives from the Korean word janchi (잔치, literally "feast" or "banquet"), because the noodle dish has been eaten for special occasions like wedding feasts, birthday parties, or hwangap (60th birthday celebration) throughout Korea. The word guksu means "noodles" in Korean, and noodles symbolise longevity - in life, in a marriage.
There are records of guksu dating back to the Goryeo period. In the book Dongguk Isangguk Jeonjip Book 6 (hangul:동국이상국전집 6, hanja:東國李相國全集) there is a mention of guksu in a line of poetry, and in the book Goryeo Dogyeong (hangul:고려도경, hanja:高麗圖經) written by an envoy from the Chinese Song Dynasty it is mentioned that guksu was eaten on special occasions as wheat was rare and expensive in Goryeo.  (The most common ingredients for noodles were buckwheat or starch.)
Because the noodles are traditionally eaten at weddings, the expression "When are you going to feed us guksu?" is a way of asking "When are you going to get married?" and a wedding day might be referred as "a day to eat guksu".