Apparently from 'anokiy; we -- ourselves, us, we.
see HEBREW 'anokiy
H587. anachnu
אֲנַחְנוּ pronoun 1 plural we see below
אֲנָ֑חְנוּ אֲנַ֫חְנוּ pronoun 1 plural common we (the plural corresponding to אָנֹכִי, as אֲנוּ; to אֲנִי; see WSG p. 100; Phoenician אנחן CISi. 3, 16.17, Aramaic ᵑ7 אֲנַחְנָא, also נַחְנָא, Syriac , , , ) Genesis 13:8; 29:4; 37:7; 42:11, 13; Numbers 9:7; Deuteronomy 1:28, 41; Joel 2:17-18, etc. Like אֲנִי, following a participle as its subject Genesis 19:13; Numbers 10:29; Judges 19:18; 2 Kings 18:26; appended to a verb for emphasis Judges 9:28; 2 Kings 10:4; Isaiah 20:6.