Definition of lang syne in English:

lang syne

adverb

Scottish
archaic
  • In the distant past:

    ‘we talked of races run lang syne’

noun

Scottish
archaic
  • [mass noun] Times gone by:

    ‘the happy lang syne’

Origin

Early 16th century: from lang, Scots variant of long + syne.

Pronunciation:

lang syne

/laŋ ˈsʌɪn/