Aparentemente, não tem base histórica.
Entrevista com Katheryn Winnick em PostCity.com
“It wasn’t originally in the script and we just wanted to come up with something unique and different. Travis [Vikings star Travis Fimmel] did it first. My new pages were revised that morning, and all of a sudden they added, ‘Lagertha licks his hand.’ When, covered with all these prosthetics, and all this alcohol and makeup on his hand, I had to lick it, I’m not kidding, 14 times.”
Wikipedia em Vikings Histórico Precisão
Little is known about Viking religious practice and so its depiction is largely creative. When Katheryn Winnick was asked why she licked the seer's hand she answered "It wasn’t originally in the script and we just wanted to come up with something unique and different". Regarding the historical differences and accuracy issues of the show, showrunner Michael Hirst comments that "I especially had to take liberties with ‘Vikings’ because no one knows for sure what happened in the Dark Ages" and that "we want people to watch it. A historical account of the Vikings would reach hundreds, occasionally thousands, of people. Here we’ve got to reach millions.
Quanto ao que significa ... parece ser apenas uma marca de respeito.