Thicken comes from thick (Old English thicce) + the verb-forming suffix -en, from Proto-Germanic *thikkwiz. The historical chain includes Old English thicce and Proto-Germanic *thikkwiz. The formation is best read as suffixal, compound, and borrowed: the word thicken carries its older form-history into a compact English lexical shape. In present English it is used as a verb, "to make or become thicker or denser". English derivatives include thickener and thickening; related English forms include thick, thickness, and thickly. The result is a verb whose etymology preserves both the older form and the later English specialization.