E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
I pledge you in this winei.e. I drink to your health or success.
Drink to me only with thine eyes,
And I will pledge with mine.
Ben Jonson (translated from Philostratus) second century.
To pledge. To guarantee. Pledging a drinkers security arose in the tenth century, when it was thought necessary for one person to watch over the safety of a companion while in the act of drinking. It was by no means unusual with the fierce Danes to stab a person under such circumstances.
Were a huge man, I should fear to drink at meals,
Lest they should spy my windpipes dangerous notes.
Great men should drink with harness on their throats.