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Phrase(s): 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.
McGrawhill's American Idioms And Phrasal Verbs