vi to become thick.
2. thicken ·vt to make thick (in any sense of the word).
3. thicken ·vt to make more frequent; as, to thicken blows.
4. thicken ·vt to strengthen; to confirm.
5. thicken ·vt to render dense; to inspissate; as, to thicken paint.
6. thicken ·vt to make close; to fill up interstices in; as, to thicken cloth; to thicken ranks of trees or men.