Geirrod's daughters are a bit, um, weird, to say the least.
The first one Thor meets, Grápr, decides to straddle the River Vímur and pee. And since she's a giantess, her urine flow is no small thing. But Grápr's probably not just peeing because she just drank six venti Frappuccinos. Nope, she's probably just loyal to dad and wants to help him best an enemy.
All of Geirrod's daughters demonstrate allegiance to Geirrod in this way, the other two attempting to crush Thor against the ceiling by giving him a chairlift ride on their backs. Geirrod certainly doesn't deserve such loyalty, but by standing with him against Thor, these girls prove themselves to be good Viking daughters.