It means "in the middle of things." The "thick" part of a situation would be the middle, where everything is happening.
There are a few different things that can be referred to when someone is described as being "thick". This could mean stubborn, unwilling to learn from experience or not exceptionally bright (or intelligent).
thick-wormed
It means that the person is slow to catch on... isn't understanding things as quickly as other people.
Laying it on thick, spreading it on thick can either mean you are exaggerating or heaping praise on someone that you expect will help you.
It is dark and thick
The three things that determine how thick or thin magma is temperature, silica content, and gas content.
"Thick" means either unintelligent or overweight, depending on the context. A person who does not learn very easy or requires to be told things repeatedly before they understand. See link for more slang terms
Things that are viscous (pours out thick) are , but are not limited to, molasses, tar, honey, etc.
How thick thick the metal is.
Thick and thin mean through good or bad like for example i am with you through thick or thin cuz ur my best friend!
thick-wormed
When you see AS ___ AS ___, you are looking at A Simile - a comparison between two things. Someone is comparing the thickness of something (you didn't say what) to a blanket. It depends on what the other thing is, actually. Blankets can be thin or thick depending on which type they are. For example, if someone said "The fog is thick as a blanket," they probably mean it's a thick, heavy fog that covers everything up.