At the stroke of midnight this New Year's, millions of people around the globe will hold hands and belt out the famous song “Auld Lang Syne” - but how many will have a clue what it means?

The Scottish poem and heartstring-tugging tune are reckoned to vie for the title of most widely recognised song on the planet after “Happy Birthday”.

From the drunken masses in London's Trafalgar Square to small towns in New Zealand, “Auld Lang Syne” is a staple of New Year's, while the tune, set to different words, appears in places as diverse as Mexico and China.

Yet with lines like “We twa hae run about the braes/And pu't the gowans fine,” it isn't only midnight tipplers who might have difficulty knowing what they are singing.

And an exhibition in New York reveals the strange-sounding words are only the tip of a murky and romantic iceberg.

“This is a song we all sort of know,” said Christine Nelson, curator of the exhibit at the Morgan Library & Museum in Manhattan. But “we don't know what it means, or where it comes from.”

The common assumption is that Scottish poet Robert Burns composed the ballad. The truth seems less straightforward.

The first recorded reference comes from Burns, who in 1788 wrote to a female friend about her recent reunion with a long-lost acquaintance.

Commenting on the theme of old friendship, Burns mentions in his letter “the Scots phrase, 'Auld lang syne,'“ which translates as “old time's sake.” Then he tells his friend: “There is an old song & tune which has often thrilled thro' my soul.”

Right there, the seeds of the world's favourite nostalgic song appear to have been planted. However, another half decade passed before Burns set the words on paper and sent them to a publisher -- and even then, it seems, almost as an afterthought.

A celebrated poet, Burns was dedicating what would be his last decade, before dying at 37, to the collection of traditional Scottish folk songs.

Dozens of songs are discussed in a long 1793 letter to the publisher George Thomson.

Then Burns mentions: “One song more & I have done - auld lang syne.”

Claiming he “took it down from an old man's singing,” Burns says the song has “never been in print, nor even in manuscript.”

He then presents the whole thing, starting with the famous lines, “Should auld acquaintance be forgot/And never brought to mind?” and continuing for a total of six verses, most of which tend to be left out by modern revellers.

Whether Burns did just copy the mysterious old man's song or whether he is in fact the secret author, is a hot subject for the poet's fans.

“My sense is that he rewrote most of it, because it really does have that ring of Burns' poetry,” Nelson told AFP.

Thomson himself thought there was “evidence of our Bard himself being the author.”

The tune took its own tortuous path. The words were first set to entirely different music, while the tune known today as “Auld Lang Syne” was originally matched to other lyrics, such as the uncatchy sounding ditty: “O Can Ye Labor Lea, Young Man.”

In 1799, when Burns was already dead, words and music were combined in their final form.

But it would be another two centuries before that potent mix of nostalgia and opportunity for a loud sing-along blew up into a global phenomenon. The song was popularised in the 1920s and '30s by American big band leader Guy Lombardo, then again in a string of Hollywood movies - notably for a crucial scene in the 1946 film “It's a Wonderful Life” - and never looked back.

How an obscure Scottish ballad with hard-to-pronounce words made that leap confounds even experts like Nelson.

“I don't know the answer to that,” she said.

What she's sure of though is that the song, which never mentions New Year's but calls for drinking to long friendship, perfectly suits the occasion.

“It's a part of almost everyone's life in the English-speaking world,” she said. “That association of valuing old friends... We understand that's what the song is about.” - AFP