Probably from 'achar; properly, deferred, i.e. The morrow; usually (adverbially) tomorrow; indefinitely, hereafter -- time to come, tomorrow.
see HEBREW 'achar
H4279. machar
מָחָר52 noun masculine, used often as
adverb, tomorrow, in time to come (Aramaic מְחַר ; probably properly time in front, Köii. 263, compare √, and Assyrian ma—ru, front, often of time, though always of former time, of old, DlHWB 402); —
1 to-morrow (the day following the present day),
a. as substantive Exodus 16:23 לְי קֹדֶשׁ שַׁבַּת ׳שַׁבָּתוֺן מָחָר to-morrow is a sabbath, etc., 1 Samuel 20:5 מָחָר חֹדֶשׁ to-morrow is the new moon, 20:18; in מָחָר יוֺם Genesis 30:33; Proverbs 27:1; Isaiah 56:12; in the idiomatic phrase מָחָר כָּעֵת at the like of the time, (being) to-morrow, = about to-morrow (see כְּ 1a) Exodus 9:18; 1 Samuel 9:16; 20:12; 1 Kings 19:2; 20:6; 2 Kings 7:1, 18; 10:6; and in לְמָחָר Exodus 8:19 הַזֶּה הָאֹת יִהְיֶה לְמָחָר, = for, against, to-morrow, 8:6; Numbers 11:18; Joshua 7:13; Esther 5:12.
b. as adverb accusative Exodus 8:25; 9:5 ׳י הַזֶּה הַדָּבָר יַעֲשֶׂה מָחָר, 10:4 אַרְבֶּה מָחָר מֵבִיא הִנְנִי, 17:9; 32:5; Numbers 14:25; 1 Samuel 11:9 etc., Joshua 11:6 הַזּאֹת כָּעֵת מָחָר; opposed to הַיּוֺם Exodus 19:10 וּמָחָר הַיּוֺם, Joshua 22:18; 2 Samuel 11:12a; rhetorically, Isaiah 22:13 נָמוּת מָחָר כִּי.
2 = in future time, Exodus 13:14 לֵאמֹר מָחָר בִנְךָ יִשְׁאָֽלְךָ כִּי = Deuteronomy 6:20; Joshua 4:6, 21; 22:24, 27, 28; so מָחָר בְּיוֺם Genesis 30:33.