Hello,
A thin card
Someone of great skill (often tinged with cunning)
An intelligent and quick-witted mind
Don’t we say that someone who is very perceptive, clever, and/or quick has a sharp mind? Sharp as a card that is pointed and well-honed…
At its origin, the expression comes from the world of fencing, where in the 16th century a fine card, initially preceded by “a good card”, denoted a good fencer.
This is easily understood, the card being that of the sword and the adjective ‘fine’ being used with its meaning of ‘excelling in an activity requiring skill and discernment’.
And just as being a fine card requires not only skill but also cunning in order to strike one's opponent at a moment when they least expect it, it is quite logical that the expression took on the figurative meaning indicated, towards the end of the 17th century.
As for the second proposed meaning, it is of course derived from the agility and intelligence of the fencing of a good fencer.
In the sixteenth century, a “veteran card” was an experienced person
Happy reading
Marylena