One way to picture it: a stretch of sand that looks exactly like the ground beside it, until you stand on it and it stops holding you up. Nothing happens fast. You go down slowly, and the harder you work, the lower you get. That combination — safe-looking, downward, and worse for struggling — is what the whole word carries.
From the word’s history
The packet traces quicksand to Old English cwecesand, quick (living) plus sand. The old sense of quick is living or moving, so the name says this sand behaves as if it were alive rather than lying still.
資料把 quicksand 追到古英文 cwecesand，也就是 quick（活的）加上 sand。quick 的老意思是「活的、會動的」，不是「快」。所以這個名字本身就在說：這片沙不像一般沙地安穩地躺著，它會動。
Wiktionary entry supplied in this packet (source_ref kaikki:3f6f69a190036ae4:499392996): 'From Middle English quyksande, from Old English cwecesand, equivalent to quick (living) + sand.'
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