With any phrase that includes a name, it's natural to consider whether its the name of a real person. In this case, as was the case with many other literary Jacks - Jack the Lad, Jack Robinson, Jack Sprat, Jack Horner, Jack Frost, etc, Jack of all trades was a generic term rather than a living and breathing individual. In fact, the very long list of terms that include 'Jack' exceeds that of any other name in English and this reflects the fact that, as a derivative of the common name 'John', 'Jack' has been used just to mean 'the common man'. This usage dates back to the 14th century and an example is found in John Gower's Middle English poem Confessio Amantis, 1390:
Therwhile he hath his fulle packe,
They seie, 'A good felawe is Jacke'.
We now use 'Jack of all trades, master of none' in a derogatory way. Originally, this wasn't the case and the label 'Jack of all trades' carried no negative connotation, the 'master of none' part being added later. Nevertheless, mediaeval Jacks were pretty much at the bottom of the social tree. The OED defines the generic meaning of the name Jack thusly:
Jack - A man of the common people; a lad, fellow, chap; especially a low-bred or ill-mannered fellow, a 'knave'
If 16th century commentators wanted to imply that a person was stretching their talents too thinly they resorted to the disparaging Latin term Johannes factotum ('Johnny do-it-all'). In 1592, the English writer and member of the literary establishment Robert Greene wrote a pamphlet entitled Groats-worth of Witte. In that he ventured the opinion that a new writer on the scene was:
An upstart crow, beautified with our feathers, that supposes he is as well able to bumbast out a blanke verse as the best of you. Beeing an absolute Johannes fac totum, is in his owne conceit the onely Shake-scene in a countrey.
Sadly for Greene's ongoing reputation the 'Upstart crow' was William Shakespeare.
Various trades were populated by Jacks - lumberjacks, steeplejacks for example, and sailors were Jack-tars. There can't have been any trades in the Middle Ages that didn't make use of a jack of some sort.
