A marlinspike with a lanyard (thanks, Wikimedia Commons)
Originally, a spike used while marling a cable, that is, wrapping twine (the marline) around a larger ropes to form protective whippings. Also used to separate strands of laid rope when splicing or forming eyes, to loosen tight knots, and as a handle for hauling small ropes (which get attached using a, wait for it, marlinspike hitch).
Thus, also, Marlinspike Hall in the Tintin books (in the English translations, anyway).
First recorded in the 1570s, and like a lot of nautical terminology from that time, it comes from Dutch. Well, the spike part doesn't (from Old Norse spīkr, nail, or other Germanic cognates), but the marling part does: Middle Dutch marlijn, small cord, from marlen, to fasten/secure (a sail), probably a frequentative of Middle Dutch maren, to tie/moor.
The attack came so suddenly that only a few sailors had gotten their cutlasses, and the rest had to make do with belaying pins and marlinspikes.