Rab. Surdam, Meaning of

The eccentric but clever and learned William Nicol, one of the masters of the High School of Edinburgh, and noted as the friend of Burns, was the son of a poor man, a tailor, in the village of Ecclefechan, in Dumfriesshire. He erected, over the grave of his parents, in Hoddam churchyard, a *throuch stone*, or altar-formed tomb, bearing the words "RAB. SURDAM." Query the meaning of these mystical characters?