To become or cause something to be thicker, broader, or denser. A noun or pronoun can be used between "thicken" and "up." Leave the soup on a low heat for another hour so that it thickens up a bit. If your batter is too runny, add a bit of flour to thicken it up. They've put me on a calcium supplement to help thicken up my bones.
thicken something up
1. to make something, such as a fluid, thicker. I have to thicken this gravy up before we can serve dinner. Please thicken up the gravy before you serve it.
2. to make something wider. See this line here? You need to thicken it up so that it shows more clearly. Try to thicken up the line a little.
1. To become thicker or denser: The gravy thickened up.
2. To cause something to become thicker or denser: I thickened the batter up by adding more flour. The cook thickened up the fudge.