Here ‘Jack’ is a name for a general labourer, one who does odd jobs and can turn his hand to anything. A ‘trade’ in this sense is a occupation – the trade of a carpenter, the trade of a bricklayer, the trade of a plumber, a mechanic, a blacksmith. Jack may have had some experience in these and other trades, yet is still master of none – that is, not a really skilled workman in any trade. The proverb therefore suggests that if you want to get on in the world, you should concentrate on one thing and learn to do it well.
