To poleaxe is to knock somebody down with one blow. Poleaxed by the news, went down poleaxed, poleaxe him.
The figurative use is the commoner, and being poleaxed by news means being completely floored by it. The weapon was a long-handled axe, and the poll in the older spelling means a head rather than a pole.
1verbpoleaxeTo knock somebody down with one blow.
“The boxer was poleaxed by a single punch in the first round.”
poleaxe is a felling.
From pollax, poll, meaning a head, plus axe, with the spelling later drawn towards pole.
From earlier pollax, from poll + axe, with the spelling influenced by pole.
Etymology from Wiktionary, CC BY-SA