Deuteronomy 9:18

18 And I felled down before the Lord as before, in forty days and forty nights, and I ate not bread, and drank not water, for all your sins which ye did against the Lord, and stirred him to great wrath; (And I fell down before the Lord as I did before, for forty days and forty nights, and I ate no bread, and drank no water, for all your sins which ye did against the Lord, and so had stirred him to such great anger;)

Deuteronomy 9:18 Meaning and Commentary

Deuteronomy 9:18

And I fell down before the Lord
In prayer for Israel who had sinned; but this he did not immediately after he had broken the tables, but when he had first ground the calf to powder, strewed it on the water, and made the children of Israel drink it; and when he had chided Aaron, and ordered the sons of Levi to slay every man his brother:

as at the first forty days and forty mights;
which is to be connected, I think, not with what goes before; for we read not that he fell down before the Lord, at the first time he was with him so long in the mount; but with what follows: "I did neither eat bread nor drink water"; as he neither ate nor drank the first forty days, so neither did he these second forty; see ( Deuteronomy 9:9 )

because of all your sins which ye sinned, in doing wickedly in the
sight of the Lord, to provoke him to anger;
for they were guilty of more sins than one; besides idolatry, they were guilty of unbelief, ingratitude which were notorious and flagrant, were done openly and publicly, in sight of his glory and majesty on the mount; all which must be very provoking to him, and on account of these Moses prayed and fasted.

Deuteronomy 9:18 In-Context

16 and I saw, that ye had sinned to your Lord God, and had made to you a molten calf, and that ye had forsaken swiftly the way of God that he had showed to you, (and I saw that ye had sinned against the Lord your God, and had cast an idol, yea, a metal calf, for yourselves, and that ye had already forsaken the way of God that he had shown you,)

17 then I threw down the tables from mine hands, and I brake those tables in your sight. (then I threw down the tablets from my hands, and I broke those tablets in front of you.)

18 And I felled down before the Lord as before, in forty days and forty nights, and I ate not bread, and drank not water, for all your sins which ye did against the Lord, and stirred him to great wrath; (And I fell down before the Lord as I did before, for forty days and forty nights, and I ate no bread, and drank no water, for all your sins which ye did against the Lord, and so had stirred him to such great anger;)

19 for I dreaded the indignation and the wrath of the Lord, by which he was stirred against you, and would do you away. And the Lord heard me also in this time praying for you (But once again the Lord listened to me praying for you).

20 Also the Lord was wroth greatly against Aaron, and would have all-broken him, and I prayed in like manner for him. (And the Lord was also greatly angered with Aaron, and would have killed him, and I prayed for him in like manner.)