矢 is a single arrow drawn upright — the head on top, the shaft in the middle, the fletching and nock (the notch that grips the string) below. If 弓 (bow) is the tool that shoots, 矢 (arrow) is what flies. The two are always paired.

Korean hanja-sound "si": 弓矢 (gungsi, bow and arrow), 嚆矢 (hyosi, a "whistling arrow") — an idiom meaning the very first instance of something. In old warfare a whistling arrow (嚆矢) was loosed to signal the start of battle, so 嚆矢 came to mean "the beginning."

Mandarin: shǐ, 3rd tone. 箭矢 (jiànshǐ, arrow), 有的放矢 (yǒudì-fàngshǐ, "to shoot with a target in view" — to act with a clear aim), 矢志 (shǐzhì, to vow firmly). Because an arrow flies straight, the senses "straight, to vow" also branched off.

Japanese: on-reading シ (shi), kun や (ya). 矢 (ya, arrow), 弓矢 (yumiya, bow and arrow), 矢印 (yajirushi, an arrow symbol), 一矢 (isshi, one arrow — 一矢報いる means "to strike back"). The 破魔矢 (hamaya) at Japanese shrines is a demon-dispelling arrow.

As a radical it marks arrows, straightness, and measuring — 知 (to know), 短 (short), 矯 (to straighten). 短 (short) comes from measuring shortness with an arrow (矢) as the ruler (尺).

Memory aid: a single arrow with head, shaft, and fletching — what flies when the bow (弓) shoots.