From 𐌲𐌹𐍃𐍄𐍂𐌰- (gistra-, compare yester-) +‎ *𐌳𐌰𐌲𐌹𐍃 (*dagis, “by day”, adverbial form of 𐌳𐌰𐌲𐍃 (dags)). Alternatively, the compound may be an adverbial form of an unattested noun *𐌲𐌹𐍃𐍄𐍂𐌰𐌳𐌰𐌲𐍃 (*gistradags, “tomorrow”) +‎ -𐌹𐍃 (-is).

The first element, which usually points to a preceding day (and not a following day, as in Gothic), has attracted some scholarly attention. It has been suggested that the meaning in Gothic shifted from “yesterday” to “adjacent day”, and thence to its singly attested meaning of “tomorrow” in Gothic. Compare also the use of the etymologically related Old Norse gær (which normally means “yesterday”) to indicate “tomorrow” in Hamðismál 30:6.

• IPA^(key): /ˈɡis.traˌda.ɣis/

𐌲𐌹𐍃𐍄𐍂𐌰𐌳𐌰𐌲𐌹𐍃 • (gistradagis)

1. (hapax legomenon) tomorrow
   • 4th century C.E., Wulfila (attributed), Gothic Bible, Matthew 6:30:

     𐌾𐌰𐌷 𐌸𐌰𐌽𐌳𐌴 𐌸𐌰𐍄𐌰 𐌷𐌰𐍅𐌹 𐌷𐌰𐌹𐌸𐌾𐍉𐍃 𐌷𐌹𐌼𐌼𐌰 𐌳𐌰𐌲𐌰 𐍅𐌹𐍃𐌰𐌽𐌳𐍉 𐌾𐌰𐌷 𐌲𐌹𐍃𐍄𐍂𐌰𐌳𐌰𐌲𐌹𐍃 𐌹̈𐌽 𐌰𐌿𐌷𐌽 𐌲𐌰𐌻𐌰𐌲𐌹𐌸 𐌲𐌿𐌸 𐍃𐍅𐌰 𐍅𐌰𐍃𐌾𐌹𐌸, 𐍈𐌰𐌹𐍅𐌰 𐌼𐌰𐌹𐍃 𐌹̈𐌶𐍅𐌹𐍃 𐌻𐌴𐌹𐍄𐌹𐌻 𐌲𐌰𐌻𐌰𐌿𐌱𐌾𐌰𐌽𐌳𐌰𐌽𐍃?

     jah þandē þata hawi haiþjōs himma daga wisandō jah gistradagis ïn auhn galagiþ guþ swa wasjiþ, ƕaiwa mais ïzwis leitil galaubjandans?

     If that is how God clothes the grass of the field, which is here today and tomorrow is thrown into the fire, will he not much more clothe you—you of little faith? (NIV)

• Lehmann, W., A Gothic Etymological Dictionary (Leiden 1986) p. 156.