矣

Bottom indexing arrow radical (Kangxi #111) - a pictograph of a feathered arrow with shaft and barb. The arrow flying away gives the perfect image for the particle's job: 矣 marks the completion of an action, the arrow already released. Same radical family: 知 (to know - words like arrows from the mouth), 短 (short, an arrow's length), 矩 (carpenter's square).

Top 厶 - a small curling shape, originally pictographic of a silkworm cocoon or a person's private enclosure. Here it sits atop the arrow as a frozen graphic element from the classical character's history; its specific function in 矣 was lost long before the modern script. Pedagogically: the cap marks the arrow as finished, sent.