KanjiJLPT N1
Rapidly

Mnemonic

An arrow zips through the air and buries itself in your shoulder. Poison seeps from its tip, and an illness floods your bloodstream rapidly. Within heartbeats the illness has spread everywhere—fever, chills, collapse. That arrow delivered sickness faster than any plague could.
Additional thoughts
Picture a black, venomous arrow embedding in flesh; illness blooms outward from the wound at impossible speed, like ink in water. Arrow + illness = something that strikes rapidly.
Quick recall
A poisoned arrow unleashes illness that overtakes you rapidly.

Details

The keyword for 疾 is rapidly. This kanji conveys the sense of swiftness and speed, often implying something that moves or occurs with great urgency. It is also closely associated with illness or disease, reflecting the idea of an ailment that strikes quickly and spreads fast. The dual meaning captures both the notion of haste and the sudden onset of affliction.
On'yomi
しつ
Kun'yomi
はや.い

Used in kanji