חוּט, H2562, is rendered “fillet” by KJV in Jeremiah 52:21 (RSV “circumference”). It clearly means a “line, thread, cord,” which could sometimes be broken easily (Judg 16:12). In this case, it refers to a line used to measure the circumference of the pillars.
M. Noth, Exodus (1962), 217.
International Standard Bible Encyclopedia (1915)
(1) Chut, from a root not used, meaning probably "to sew," therefore a string or a measuring rod or cord, and so a line, tape, thread, fillet. Jer 52:21 translated "line" (the King James Version "fillet"), measuring 12 cubits long, encircling brass pillars standing 18 cubits high, part of the temple treasure plundered by the Chaldeans; and many other things "that were in the house of Yahweh, did the Chaldeans break in pieces." Translated "thread," used by Rahab, in Jos 2:18, and "cord," "three fold .... is not quickly broken," in Ec 4:12.
(2) Chashuq, from a root meaning "to join" and therefore something joined or attached, and so a rail or rod between pillars, i.e. a fillet. The hangings of the court of the tabernacle were supported by brass pillars set in brass sockets, "The hooks of the pillars and their fillets shall be of silver" (Ex 27:10,11). The embroidered screen for the door of the Tent was supported by five pillars socketed in brass: "And he overlaid their capitals and their fillets with gold" (Ex 36:38). The pillars for the court and the gate of the court had fillets of silver (Ex 38:10 ). The verb is used in Ex 27:17; 38:17, "All the pillars of the court were filleted with silver."