In this paper, we define
the concept of topological left thick subsets in a locally compact semigroup
which is a generalisation and extension of the concept of left thick subsets in
(discrete) semigroups introduced by T. Mitchell and prove that if T is a Borel
measurable subset of a locally compact left amenable semigroup S, then T is
topological left thick if and only if there is a topological left invariant mean
M on S such that M(χT) = 1 where χT is the characteristic functional of
T in S, thus generalising, and extending a result of Mitchell for (discrete)
semigroup.