To make thick (in any sense of the word). Specifically:
To render dense; to inspissate; as, to thicken paint.
To make close; to fill up interstices in; as, to thicken cloth; to thicken ranks of trees or men.
To strengthen; to confirm. And this may to thicken other proofs. (Shak)
To make more frequent; as, to thicken blows.
Origin: Thickened; Thickening.