A term of praise to signify wine of superior quality. A notion prevails that the grapes in comet years are better in flavour than in other years, either because the weather is warmer and ripens them, better, or because the comets themselves exercise some chemical influence on them. Thus, wine of the years 1811, 1826, 1839, 1845, 1852, 1858, 1861, etc., have a repute.
“The old gentleman yet nurses some few bottles of the famous comet year (i.e. 1811), emphatically called comet wine.” —The Times.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894