A lock which has no spring catch. Metaphorically, a state of
things so entangled that there seems to be no practical solution.
“Things are at a dead-lock.” —The Times.
Empty bottles. Down among the dead men let me lie. Let me
get so intoxicated as to slip from my chair, and lie under the table
with the empty bottles. The expression is a witticism on the word
spirit. Spirit means life, and also alcohol (the spirit of full
bottles); when the spirit is out the man is dead, and when the bottle
is empty its spirit is departed. Also, a loaf of bread smuggled into
the basket for the private use of the person who carries the bread out
is called a “dead man.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Dead Lock from Infoplease: