A distaff is a tool used to spin fibers into flax or wool. The oldest type of distaff was held under one arm as the spinner slowly unwound fibers from it and onto a spindle in the opposite hand.
Spinning wool is an old-fashioned craft, so it's not common to see a distaff in use these days. Some are attachments on spinning wheels, while others are turned by hand. As it was traditionally women who wielded distaffs, the word came to be used to mean "women's work" in general, as early as the 14th century. Today it's also an adjective meaning "concerning women." So instead of planning a girls' night out, your sister might instead arrange a "distaff gathering."