One way to hold it: a line drawn on the ground, and something suddenly coming in across it from the other side. The word names that one movement inward — not the staying, not the going out again, just the crossing in.
可以想成地上畫了一條線，然後有東西突然從外面越過那條線進來。這個字指的就是「進來」那一下：不是留下來，也不是退出去，而是跨過那條界線的動作本身。
From the word’s history
From Middle English, borrowed from Old French, from Latin incursiō. The word came into English already meaning a running in upon someone, and the modern senses all keep that one inward movement.
從中古英文往前推，是借自古法文，再往前是拉丁文 incursiō。它進到英文時就帶著「衝進來」的意思，今天的每一個用法都還留著那個往內的方向。
Wiktionary entry supplied in this packet (source_ref kaikki:3f6f69a190036ae4:516053251), etymology_text: From Middle English, borrowed from Old French, from Latin incursiō.
登入就可以看每一支怎麼連回核心畫面。