Webster's Revised Unabridged Dictionary · 1913

Thicken

Thicken , transitive verb

To make thick (in any sense of the word).
(a)
To render dense; to inspissate; as, to thicken paint.
(b)
To make close; to fill up interstices in; as, to thicken cloth; to thicken ranks of trees or men.
(c)
To strengthen; to confirm. [Obsolete]
And this may to thicken other proofs. — Shakespeare
(d)
To make more frequent; as, to thicken blows.

Thicken , intransitive verb

To become thick.
Thy luster thickens when he shines by. — Shakespeare
The press of people thickens to the court. — Dryden
The combat thickens, like the storm that flies. — Dryden