Exodus 10:21

וימש חשך signifies and it (the darkness mentioned in the preceding phrase, ויהי חשך) shall darken for them the natural darkness to a higher degree than the darkness of night: i. e. the darkness of night shall become even more black and dark. וימש means as much as (is a contraction of) ויאמש (a Hiphil form of אמש, evening darkness). We have many words in which an א is omitted; since the sound of the א is not very marked, Scripture is not particular about omitting it. An example is: (Isaiah 13:20) “the Arabian לֹא יַהֵל there”, which is the same as לא יַאֲהֵל, shall not pitch his tent. Similar is, (II Samuel 22:40) “וַתַּזְרֵנִי with strength”, which is the same as וַתְּאַזְרֵנִי “And thou hast girded me” (cf. Psalms 30:12). Onkelos translated it in the sense of removing, from the same root as, (Joshua 1:8) “it shall not depart (ימוש)”, his rendering being, “[and there shall be darkness..] after the darkness of night has departed”, i. e. when the time approaches near to the light of day. But the statement does not then fit with the ו of וימש, because this word is written after ויהי חשך. A Midrashic statement explains it in the sense of (Deuteronomy 28:29) “groping (ממשש feeling) at noon-day”, so that the meaning would be that it was of such a double character and so thick that there was something tangible in it (Exodus Rabbah 14:1-3).

וַיֹּ֨אמֶר יְהֹוָ֜ה אֶל־מֹשֶׁ֗ה נְטֵ֤ה יָֽדְךָ֙ עַל־הַשָּׁמַ֔יִם וִ֥יהִי חֹ֖שֶׁךְ עַל־אֶ֣רֶץ מִצְרָ֑יִם וְיָמֵ֖שׁ חֹֽשֶׁךְ׃

And the Lord said to Moshe, Stretch out thy hand towards heaven, that there may be darkness over the land of Miżrayim, darkness which may be felt.