oakum n.

"coarse, loose fiber obtained from taking apart old hemp ropes," used for caulking the seams of wooden ships, etc., early 15c., okam, okum, from Old English acumba "tow, oakum, flax fibers separated by combing," literally "what is combed out," from Proto-Germanic *us-kambon (source of Old High German achambi). The first element is cognate with Old English a- "away, out, off;" the second element is from stem of cemban "to comb," from camb "a comb;" from PIE root *gembh- "tooth, nail."