I had to look this up. At 38 years old, I had never heard this term. Apparently it was appropriated by women, and the original one (which makes a lot more sense) is 'Eskimo Brothers'. The idea is that the 2 of them shared the same igloo, forming a bond as brothers. The igloo in this case being a vagina.
Although, I suspect the real definition is related to eskimo hunting and gathering in pairs and sharing in a kill, but I couldn't find a reliable source for that (only hints here and there). I think the igloo one was just made up because it sounded edgier. The term was showing up in literature as far back as the late 1700s, so it just seems unlikely to have such a modern nuance.
Welp, that was a good way to spend my lunch break. A little dog fucking, a set of the most massively deformed breasts I've ever seen, and an epilogue of etymology. TGIF.