As in, courage derived from the imbibing of alcohol.

Wikipedia takes a positive line, saying that the idea of it being perjorative propaganda is wrong. Instead, it suggests the Dutch were a feared naval power in the 17th century, and their ferocity must have been fuelled by drink. A letter in the Times says it was traditional for a barrel of gin to be placed before the masthead prior to battle on Dutch ships.

An alternative is that the Dutch traders coming to London during the Great Plague used to drink alcohol to fend off illness.

Most likely is that gin was a popular drink, and was associated with Holland.