糸 is the shape of fine threads twisted into one bundle — the top is the twisted skein, the three small dots below are the loose thread-ends. A single strand of silk drawn from a cocoon is its core sense. Double it and you get 絲 (silk).

Korean hanja-sound "sa" (also "myeok"): it is used far more as a radical than as a word on its own. Almost every character about silk and twisted thread takes the 糸 radical.

Look at the characters built with 糸 and a whole world of thread unfolds: 線 (line), 細 (fine/thin), 絹 (silk), 結 (to tie), 終 (to finish), 綠 (green), 紙 (paper), 約 (to promise), 續 (to continue) — all grow from thread. Finishing (終), continuing (續), and even promising (約) were originally matters of binding with thread.

Mandarin: mì / sī. As a radical it shrinks in simplified Chinese to 纟 (three strokes) on the left — 线 (線), 红 (紅), 级 (級). Japanese: on-readings シ (shi) and ベキ (beki), kun いと (ito). 糸 (ito, thread), 毛糸 (keito, wool yarn), 糸口 (itoguchi, a clue — "the mouth of the thread" is the lead into a matter). Japanese 糸 (ito) refers to thread in general.

Memory aid: a twisted skein with three loose ends below — a single silk strand, the root of every thread character.