From The Collaborative International Dictionary of English v.0.48 [gcide]:
Sponger \Spon"ger\ (-j[~e]r), n.
1. One who sponges, or uses a sponge.
[1913 Webster]
2. One employed in gathering sponges.
[1913 Webster]
3. Fig.: A parasitical dependent; a hanger-on.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
sponger
n 1: a workman employed to collect sponges
2: a follower who hangs around a host (without benefit to the
host) in hope of gain or advantage [syn: leech, parasite,
sponge, sponger]