An Etymological Dictionary of the German Language/Tau
Tau (1.), n. (unknown to Swab. and Bav.), ‘rope, cable,’ ModHG. only; properly a LG. word, based on OIc. taug, ‘cord, rope’ (whence E. tow, Du. touw). The latter is connected with the Teut. root tuh (tang), in ModHG. ziehen. From the LG. word Fr. touer, ‘to tow a ship,’ is derived. For the words borrowed by HG. from LG. see Strand, Boot, &c.
Tau (2.), m., ‘dew,’ from the equiv. MidHG. and OHG. tou (gen. touwes), n. (MidG. also, m.); corresponding to the equiv. OSax. dau, Du. dauw, AS. deáw, E. dew, OIc. dǫgg (Goth. *daggwa is wanting), whence E. (dial.) dag. Teut. dauwo-, from pre-Teut. dháwo-, is generally connected with the Sans. root dhā̆v, ‘to run, flow, stream.’