Parashas Bo · Passuk 10:21
Chabad.org — English translation + Rashi
Torah Temimah points to Mechilta de-Rabbi Yishmael, Tractate Vayehi Beshalach (section on the darkness) (verified).
English: “A darkness that can be felt” — Scripture tells that when an Egyptian was standing he could not sit, and when he sat he could not stand, because he was groping in the darkness. And the darkness was as thick as a dinar.
In context: See the cited source for the full section.
Derash on the verse (Mechilta) — lexical and physical reading of the word vayamesh as palpable darkness.
The darkness of Egypt was palpable: it held people where they stood or sat, and its density is compared to the thickness of a coin.