The answer to this question can be found in the first chapter of this great novel, which details how Jessie was kidnapped by the sailors and taken to the ship. As he walks back home, dreaming of the possible future he might have, he is greeted by a very different reality as he finds his head covered with a sack and he is dragged away. Note how this experience is described:
What I encountered was foul smelling canvas, a sky full of it, covering me entirely, forcing me to the ground.
In addition, the cloth is later said to be "musty" in a way that prevents Jessie from crying out. Thus the text does not give us a direct answer to your question. We know that the sack is "foul-smelling," but we are not told of what the sack smells. However, given the nature of the sailors and the fact that they are trying to kidnap Jessie, we can imagine that the sack probably is old and perhaps has been used for other kidnap attempts or to carry other, less savoury things.
"What I encountered was foul smelling canvas, a sky full of it, covering me entirely, forcing me to the ground" "I tried to shout but the musty cloth filled my mouth and I could get no air into my lungs" (pg 13)