I hadn't heard that expression so I googled it:

Snakes in his Boots (To have).

To suffer from D.T. (delirium tremens). This is one of the delusions common to those so afflicted.

1. “He’s been pretty high on whisky for two or three days, … and they say he’s got snakes in his boots now.”—The Barton Experiment, chap. ix,

CITATION: Brewer, E. Cobham. “[Entry Title].” Dictionary of Phrase and Fable. Philadelphia: Henry Altemus, 1898; Bartleby.com, 2000. www.bartleby.com/81/