To be sleighed is to be immeasurably drunk on Christmas Eve, so much so that you are either still drunk Christmas Day or are massively hung over. The same as "wasted" or "shit-faced" but used specifically on the Christmas Holiday. This term can also be used post-Christmas if one is wasted Christmas Day and continues into Boxing Day.
This is often seen in "mall Santas" after they finish thier final day on the 24th.
Guy 1: "Whoa, did you see that mall santa tossing his guts under that tree?"
Guy 2: "Yeah, he's totally sleighed!"
Santa: Whooaaa there Rudolph, I said leeeeft.... hold up I gotta ho ho ho hurl! *blargh*
Rudolph: Good graces Santa! Your F*cking Sleighed!