Istapp (plural istappar) = icicle. Another Swedish word for icicle is ispigg (the SAOB definition of istapp is simply ispigg), where pigg = spike, quill, as discussed previously. So what about tapp?
As usual, here things get somewhat confusing. SAOB says tapp has a Germanic origin, meaning tapered. However, according to Oxford, taper is derived from the latin papyrus, and refers to the shape of a candle, the wicks of which were made from papyrus pith. Tapp also means tap, in the sense of the spout part of a tap; the whole appliance (faucet) is a kran. So you can see that they all seem to be linked together somehow.
The verb tappa tells you many things that can be done with a tapp, such as tappa upp ett bad, run a bath.
Etymology aside, other nice tapering thing is a molntapp, wisp of cloud.
But by far the most curious word on this page must be icicle, which is apparently derived from ice + ickle, where ickle is an English dialect word meaning icicle. Go figure! Or, beware of etymology: