Sentence:  Let X be an acyclic 2-complex and M^3_0 an abstract regular neighborhood of X; \partial M_0=S^2, so <WSD>cap off</WSD> to get a homology 3-sphere M^3. 
Meaning of the word marked with <WSD> tags:  To glue a relatively uncomplicated compact manifold to a manifold of the same dimension along (a component of the latter's boundary).