This one has two meanings - 1) If the person who said it is from Newfoundland, Canada, then it means to take your fill of something, like "help yourself to as much as you want". 2) If you are making refernce to someone filling their boots, it refers to what would happen if you pooped your pants severely (from a fright), it would flow down and fill your boots.
Wetting your pants and "filling your boots" after a fright - a phrase used to refer to someone who isn't very smart: "Couldn't pour p*ss out of a boot with the directions on the bottom."
http://www.phrases.org.uk/bulletin_board/7/mes sages/560.html
Exclam. Help yourself! Enjoy yourself!
Vrb phrs. Be very scared.
http://www.peevish.co.uk/slang/f.htm
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...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
1. Invite another to help themselves to their optimal capacity or
2. Attempt a difficult task or ordeal.
3. An invitation to partake with gusto, as in "be my guest," or "help yourself."
http://www.urbandictionary.com/define.php?term =fill+your+boots
At the H