This is going to bring me large numbers of search engine hits from people looking for something else entirely, but I’m going to talk about the joy of X, which inevitably means mentioning XX, and XXX of course, and XXXX and so on, right up to Simonds of Reading’s strong stout, Archangel XXXXXXX.
The usual (and only semi-likely) explanation of the original use of X and XX as markings on ale and beer casks, and subsequently as beer names, was that they were used as a guarantee of quality by monastic brewers: Frederick Hackwood’s Edwardian-era Inns, Ales and Drinking Customs of Old England says that
in shape the crosses were at first more akin to the crucifix, and served to indicate that by the oath of the monks, ‘sworn on the cross’, the beer was of sound quality, fit to drink.”
though, of course, there is no contemporary documentary evidence given for this, and it seems unlikely, frankly, that monks would use Christianity’s holiest symbol on casks of ale. In any case, † is † and X is X.
Another explanation is that it comes from the habit of excisemen from the middle of the 17th century, when beer was first taxed, marking XX on casks of strong ale or beer and X on casks of small beer. The problem with that idea is that the excisemen’s marks were X for strong beer and T for table beer.