Charles Ellicott Commentary Exodus 10:21

"And Jehovah said unto Moses, Stretch out thy hand toward heaven, that there may be darkness over the land of Egypt, even darkness which may be felt." — Exodus 10:21 (ASV)

Darkness which may be felt. —Hebrew: one shall grasp darkness. The Authorized Version seems to give the true meaning, which is found also in the Septuagint and the Vulgate. The idea is an exaggeration of that instinctive feeling which makes us speak of “thick darkness.” The general voice of mankind confirms the use of the phrase.