Wikipedia: "A hectocotylus (plural: hectocotyli) is one of the arms of male cephalopods that is specialized to store and transfer spermatophores to the female.[1] Structurally, hectocotyli are muscular hydrostats. Depending on the species, the male may use it merely as a conduit to the female, or he may wrench it off and present it to the female."

In other words, they're named after a squid's dick. Which some species just rip off and give to the female, so she can fuck herself with it.