Verb

(en verb)

To become hard (tough, resistant to pressure).

(ergative) To make something hard or harder (tough, resistant to pressure).

(dated) To become or make a person or thing resistant or less sensitive.

When thou goest to return into Egypt, see that thou do all those wonders before Pharaoh, which I have put in thine hand: but I will harden his heart, that he shall not let the people go. — KJV, Exodus 4:21

Verb

(en verb)

To make thicker (in the sense of wider).

To make thicker (in the sense of more viscous).

To become thicker (in the sense of wider).

* {{quote-magazine, year=2013, month=July-August, author= Stephen P. Lownie], [http://www.americanscientist.org/authors/detail/david-m-pelz David M. Pelz

, magazine=(American Scientist), title= Stents to Prevent Stroke , passage=As we age, the major arteries of our bodies frequently become thickened with plaque, a fatty material with an oatmeal-like consistency that builds up along the inner lining of blood vessels. The reason plaque forms isn’t entirely known, but it seems to be related to high levels of cholesterol inducing an inflammatory response, which can also attract and trap more cellular debris over time.}}

To become thicker (in the sense of more viscous).

To strengthen; to confirm.

* (William Shakespeare)

And this may help to thicken other proofs.

To make more frequent.