What is the meaning of Quid?

The inherent nature of something.

A member of a section of the Democratic-Republican Party between 1805 and 1811, following John Randolph of Roanoke. (From tertium quid.)

Paired with quo, in reference to the phrase quid pro quo (this for that): something offered in exchange for something else.

A sovereign or guinea, that is, a certain coin or amount of money.

Pound sterling.

Various national currencies typically known by the name "pound".

Euro.

Dollar, dollars.

A piece of material for chewing, especially chewing tobacco.

The act of chewing such tobacco.

To chew tobacco.

To let food drop from the mouth whilst chewing.

crux, gist, core

what about

a certain something (that is somehow undefinable)

neuter nominative/accusative singular of quis

what, how?

why? what for?

well, why, what?

gist; point; crux

Source: wiktionary.org