I. (ˌ)laŋ-ˈzīn, -ˈsīn adverb
Etymology: Middle English (Scots), from lang long + syne since
Date: 15th century
chiefly Scottish : at a distant time in the past
II. noun
Date: 1694
chiefly Scottish : times past
should auld acquaintance be forgot, and days o' auld lang syne — Robert Burns