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Someone who dampens enthusiasm: “We were all having a good time until Harold walked in and started acting like a wet blanket.”
A Mexican who enters the US illegally, esp as a migratory worker •The term may be generalizing to include all illegal immigrants: An American's as good as a wetback, who is a Mexican whom we don't know how he got here
[1929+; fr the fact that they get their backs wet in wading across the Rio Grande; the terms wet pony, wet cow, etc, were used earlier for animals brought illegally across the border]