oakum
noun
oa·kum ˈō-kəm 
: loosely twisted hemp or jute fiber impregnated with tar or a tar derivative and used in caulking seams and packing joints
Example Sentences
Recent Examples on the Web Longtime DIY owners talk about pouring cement between the foundation rocks and buying oakum, sailor's tarry rope, to weave it between the logs to keep the wind out. 
Janet Eastman, OregonLive.com, 6 July 2017
Janet Eastman, OregonLive.com, 6 July 2017 Word History
Etymology
Middle English okum, from Old English ācumba tow, from ā- (separative & perfective prefix) + -cumba (akin to Old English camb comb) — more at abide
First Known Use
15th century, in the meaning defined above