Probably from an unused root meaning to be narrow; according to most a plumb-line, and to others a hook -- plumb-line.
H594. anak
אֲנָךְ noun [masculine] plummet (compare words in cognate languages for lead, tin, etymology dubious, perhaps foreign; Arabic (see Frä153) Syriac Mandean אנכא, Assyrian anâku LyonSargontexte 92; see LagArm. Stud. § 103 compare Id.BN 175) — אֲנָ֑ךְ except once Amos 7:8 — plummet 7:7-8, (twice in verse); א ׳חוֺמַת, i.e. a vertical wall vAmos 7:7.