From Wikipeda - http://en.wikipedia.org/wiki/Jock_Tamson's_Bairns 

 

"We're a' Jock Tamson's Bairns" (Lowland Scots for we're all John Thomson's children) is a popular saying in Scotland and the far north of England, and is known in other parts of the world. Nowadays, the phrase is often used to mean "we're all the same under the skin".It has been suggested as a euphemism for God, so the saying could mean "we are all God's children". The expression "We're a' the bairns o' Adam", conveys exactly the same meaning, see Freedom Come-All-Ye a song written by Hamish Henderson. Scottish Gaelic also has the shorter saying "Clann MhicTamhais" (Thomson/MacTavish's children/clan). This is a common egalitarian sentiment in Scottish national identity, also evident in the popularity of the Robert Burns song A Man's A Man for A' That.Although Jock Tamson's Bairns is used as a personification of the Scots nation, it is also used to refer to the human race in general.[1]It is also used when people think one of their number is showing off, or considers himself better than his peers: "Who does he think he is? We're all Jock Tamson's bairns."[2] The downside of this egalitarianism is the traditional lack of acceptance of anyone from a small community who moves on and up, socially or professionally, even if they display no conceit. "Too good for us now, are ye?"One explanation of this phrase (as recorded in the History of Duddingston Kirk) is that the Reverend John Thomson (Jock Tamson, Thamson), minister of Duddingston Kirk,Edinburgh, from 1805 to 1840, called the members of his congregation "ma bairns" (cognate with Geordie me bairns; English: 'my children') and this resulted in folk saying "we're a' Jock Tamson's bairns" which gave a sense of belonging to a select group."Jock Tamson" (John Thomson) would have also been a very common Scottish name, and would have been equivalent to such phrases as "John Doe", "John Smith", "Joe Bloggs" etc.