A simplicial set $X$ is $k$-coskeletal iff the following condition holds:
a simplex of dimension $> k$$\geq k$ is present iff all of its $k$$(k-1)$-dimensional faces are present in $X$.
A standard example is the usual nerve of a small category, which is readily seen to be $1$$2$-coskeletal. (Similarly, the nerve of an $n$-category is $n$$(n+1)$-coskeletal).