From the terminology of glassmaking; a prunt is a small blob of glass applied to the surface of a glass object.
The date of origin of the prunt is not clear but it is commonly believed that prunts first appear on glass steins made in Northen Germany, circa 14th Century. At first, they were simply crude blobs of molten glass, dropped from the end of a rod onto the outer surface of a stein. It is thought that they were intended to offer the drinker a good grip on his glass as the use of cutlery wasn't wide-spread in these times and diners hands were often greasy. In Germany, prunts were known as Nuppen, which means "drop". The drinking vessels to which they were applied were known as Warzenbecher, ("Wart beaker"), or, simply, Nuppenbecher.
By the 19th Century, the lowly prunt was considered quite old-fashioned as a decorative device and had been rendered redundant by the introduction of handles to drinking vessels. However, like all unfashionable things, the prunt went so far out of fashion that it eventually came back in fashion, (in the late 19th Century). At this stage, their use was purely decorative, with the prunt perhaps being of a different colour to the glass it was attached to. Prunts were also formed more precisely and were often impressed with some kind of pattern whilst the blob of glass was still malleable. Sometimes more elaborate designs appeared, with the "common or garden" blob-like prunt transformed into a human figure, or an animal perhaps.
Prunts are still around today, mostly on the shoulders of a small number of wine and liquor bottles where they are often impressed with some kind of seal or stamp.