[Can_Not]

Also smug is less specific than knowledge, it's more about thinking you're better (in some or all ways) or more well achieved. The opposite of humble.

[b2gills]

I would say that smugness has almost nothing to do with knowledge, and everything to do with attitude.

I have seen people being smug that they know nothing about a given subject.

That is smugness is almost exclusively about “thinking you're better…”.

If someone thinks they have more knowledge, or thinks they have achieved more; it is easier for them to become smug in that belief. People can also become smug even if they know that neither is true.