TSUMUGU · TOCFL 1 (est.) · 字
套
ㄊㄠˋ · tào
noun/verb/measure-word — a cover, sheath; a set; to slip over; (m.w.) a set of
字源 FORM — what the parts do
大 (big) over 長 (long): something opened wide and drawn out to slip over another thing the whole length of it — a cover, a sheath (手套、封套). What goes over as one piece becomes a set (一套、套裝、成套), and as a measure word, 一套 counts the set. The same "fit one over another" gives the figurative 套話、圈套 — set phrases, a trap laid to drop over you.
故事 STORY — a scene to remember it by
把口張得又大（大）又長（長），整個罩下去——一套就套上了。
Opened big (大) and long (長), dropped over the whole length — and 套 has it covered.
字源記憶法
意 · Meanings
(本義)套子、手套— a cover, sheath常用
→一套、套裝— a set; a suit (also a measure word)常用
⇒圈套、套話— a trap; set phrases
(本義) original meaning · → ⇒ each arrow = one more step of extension