Probably from an unused root meaning to be narrow; according to most a plumb-line, and to others a hook -- plumb-line.
Englishman's Concordance
1) plummet, plumb, lead-weight Part of Speech: noun masculine A Related Word by BDB/Strong’s Number: probably from an unused root meaning to be narrow Same Word by TWOT Number: 129.1
Brown-Driver-Briggs
אֲנָךְnoun [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 — plummetAmos 7:7,8 (twice in verse); ׳חוֺמַת א, i.e. a vertical wall vAmos 7:7.
Amos 7:7 HEB:עַל־ חוֹמַ֣ת אֲנָ֑ךְ וּבְיָד֖וֹ אֲנָֽךְ׃ NAS: was standing by a vertical wall KJV: upon a wall [made] by a plumbline, with a plumbline INT: and wall A vertical his hand A plumb
Amos 7:7 HEB:אֲנָ֑ךְ וּבְיָד֖וֹ אֲנָֽךְ׃ NAS: wall with a plumb line in His hand. KJV: [made] by a plumbline, with a plumbline in his hand. INT: A vertical his hand A plumb
Amos 7:8 HEB:עָמ֔וֹס וָאֹמַ֖ר אֲנָ֑ךְ וַיֹּ֣אמֶר אֲדֹנָ֗י NAS: And I said, A plumb line. KJV: thou? And I said, A plumbline. Then said INT: Amos said A plumb said the Lord
Amos 7:8 HEB:הִנְנִ֨י שָׂ֤ם אֲנָךְ֙ בְּקֶ֙רֶב֙ עַמִּ֣י NAS: A plumb line. Then the Lord KJV: Behold, I will set a plumbline in the midst INT: I am about to put line the midst of my people