From where do we get the expression 'fill your boots', meaning to have as much as you want?

A suggestion was that the phrase originated with the English Cavaliers, who wore thigh-high riding boots. When drinking, rather than stepping outside to relieve himself, a Cavalier apparently had the option of doing so into his boots. Thus, "filling his boots" meant he could drink all he wanted without leaving the table.

http://plateaupress.com.au/wfw/fillyour.htm

Sorry princess, not the answer I was looking for.

On Sunday it was mentioned by one of the experts on the Antiques Roadshow. Someone had brought in a silver inlaid leather flask and was told that in Nelson's time such vessels were known as 'boots'. To have as much wine as they wanted they were told to 'fill your boots'. I must admit though, I can't find any other reference to it.

There seems to be several sites that mention that it originated with English Cavaliers who wore thigh-high riding boots. When drinking, rather than stepping outside to relieve himself, a Cavalier apparently had the option of doing so into his boots. Thus, "filling his boots" meant he could drink all he wanted without leaving the table. This has been turned into an annual theatrical event, with slight variation to the storyline, which is shown around Christmas time called P*ss in Boots

At the HMS Victory museum in Portsmouth UK, you can buy a thick leather cup lined with pitch. This is a replica of the sailor's mug used on board in Nelson's time, and it was used (among other things) for the rum ration when issued. This cup is called a "boot", and when things were good and you got an extra rum ration, sailors were told "Fill Yer Boots"!