What does fill your boots mean?

Not in the Official Urban Dictionary, from http://plateaupress.com.au:

Different meanings:

Pat H I have often heard the phrase “fill your boots” used to mean “have at it” or “take/do all you want”, “have your fill”, etc. While the phrase does have other meanings, I’m interested in the origin of this one. I read one reference which suggested 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. Gross, but is it true?

Frank Pierce I’ve heard (and sometimes use) another definition that’s a few degrees off from “have at it”. To fill one’s boots means to serve as a replacement, to stand in someone’s stead. E.g., “Can Mr. A fill Mr. B’s boots?”

Pat H Forgot to mention that I’m from Canada (oh, Canada…) eh. This may be a Canadianism but I doubt it. AtoZ I’m told that “fill your boots” in the sense of “go ahead, help yourself” is a Newfoundland phrase. I’ve also read that the same phrase has been used by a UK sportscaster to mean “score a lot of goals”.

Since I live neither in Newfoundland nor the UK I cannot confirm either.