allowing him no purchase

Kazuo Ishiguro氏のThe Buried Giantを読んでいます。
The baskets were moving ever more slowly now, pulling inwards towards the sludge-like water where the bank made its turn. Thrusting the staff into the water, Axl found he could touch the bottom easily, but when he tried to push off back into the tide, the river floor sucked at the stick, allowing him no purchase.
引用文最後の個所に出てきた "purchase" は10/4/2017に覚えたばかりの意味ですね。前回とは別の辞書の説明を引用します。
・Cambridge English Dictionary: a firm hold that allows someone or something to be pulled or lifted without sliding or falling: Dancers use a special powder on their shoes to help them get a better purchase on the floor.