Think of a fingertip moving towards a soft spot — the side of the ribs, the sole of a foot. Nothing has happened yet, and the body is already reacting: a laugh, a jump, a pulling-away. That is the whole picture inside ticklish. The touch is light, the spot is the kind that answers back, and the answer is immediate and out of the person's control. Every use of the word keeps those three things: light contact, a place that is easily set off, and a reaction you cannot stop.
From the word’s history
Wiktionary derives ticklish from the verb tickle plus the ending -ish, which often means 'having that quality, easily like that'. So the word starts out meaning literally 'easily tickled', which is exactly the picture above.
依 Wiktionary 的說明，ticklish 由動詞 tickle（搔癢）加上字尾 -ish 組成，-ish 常表示「帶有那種性質、容易那樣」。所以這個字一開始的字面意思就是「容易被搔到癢的」，正好就是上面那個畫面。至於後來用在事情上的「棘手」，是一種好記的延伸想法，這裡不當成歷史演變的主張。
Wiktionary, "ticklish"
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