stock still

Miyuki MiyabeのCrossfireを読んでいます。
第4章の冒頭文を引用します。
Junko Aoki stood stock still at the end of a dead-end alleyway and hugged herself as best she could to keep them.
"stock still" の意味を辞書で調べます。
・Oxford English Dictionary: Without any movement; completely still.: he stood stock-still
・Collins Dictionary: If someone stands or sits stock-still, they do not move at all.: The lieutenant stopped and stood stock-still.

・Cambridge English Dictionary: without moving; completely still: On seeing us, the deer stood stock-still for a moment, then turned and retreated into the forest.
Etymology Onlineに次の記述がありました。
Stock-stock (late 15c.) is literally "as still as a tree trunk." ...Stock "stick, cane"