He made a dead set at her. A pointed or decided determination
to bring matters to a crisis. The allusion is to a setter dog that has
discovered game, and makes a dead set at it.
To be at a dead set
is to be set fast, so as not to be able to move. The allusion is to
machinery. To make a dead set upon someone
is to attack him
resolutely, to set upon him; the allusion being to dogs, bulls, etc.,
set on each other to fight.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894