The Robert Burns (or Bobby/Bobbie Burns) is one of those classic cocktails from the golden age of adult beverages. Most people would associate the drink with Robert Burns, the famed Scottish poet of the 1700's. According to A.S. Crockett's "The Old Waldorf-Astoria Bar Book", it may have been named after a cigar salesman who was a regular at the bar.
The drink itself is a delightful, sophisticated mix that builds on the Scotch-vermouth combination of the Rob Roy. The addition of absinthe brings in a nice contrasting flavor, though be sure to keep this accent to a dash, as it can easily overpower the drink.
It is a Scottish tradition to celebrate the poet Burns' birthday on January 25th of each year during what is known as Burns Night. You can read more about Robert Burns and Burns Night here: What Is Burns Night?