A primitive root; probably to glow, i.e. (figuratively) be desolated -- be darkened.
Englishman's Concordance
1) (Niphal) to be burned up, be scorched 1a) meaning dubious Part of Speech: verb A Related Word by BDB/Strong’s Number: a primitive root Same Word by TWOT Number: 1720
Brown-Driver-Briggs
[עָתַם] verb dubious;
Niph`alPerfect3masculine singular נֶעְתַּםאָ֑רֶץIsaiah 9:18 (is be clogged in speech, suffocating heat, burned by heat; hence Thes scorched, but derived sense dubious; see RSJPhil. xiii. 61 f.); — read perhaps נִצְּתָה (√ יָצַת) is burned up, compare RSl.c., Krochm Kit CheHpt.
Isaiah 9:19 HEB:יְהוָ֥ה צְבָא֖וֹת נֶעְתַּ֣ם אָ֑רֶץ וַיְהִ֤י NAS: the land is burned up, And the people KJV: is the land darkened, and the people INT: God of hosts is burned the land become