A Jack of all trades is a master of none 樣樣通樣樣稀鬆；博藝無所精
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If you are able to do a lot of things fairly well, you will not have time to learn to do one thing extremely well.
The phrase ‘Jack of all trades master of none’ means that the person knows how to do many things, but is not considered a master of a specific field. For instance, a person can be good with fixing things, making sketches and building simple furniture, but he is not an expert on any of them. So, in short, it means a person who knows some stuff but is not considered an expert at anything.
We now use ‘Jack of all trades is a 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’
