dago T n. a ribbed, white cotton tank-style undershirt; a wife-beater; a boy-beater. Editorial Note: A synonym is guinea T, which, like dago T, is often perceived as derogatory or offensive. (source: Double-Tongued Dictionary)
Further reading
Unparalleled misalignments are pairs of phrases in which the words in one phrase are each synonyms of the words in the other, but the phrases themselves mean different things. For example, the phrase blanket statement can be paired with cover story...
Here’s an unparalleled misalignment in which the words in one pair are synonyms of the words in the other pair: organic chemistry and carbon dating. This is part of a complete episode.
Related
Found a new instance to add to definition, used as ‘obscene slang’ against a Mr.E :
“That’s what E is—a DAGO!”
He nodded like a man who gives a last tap to a nail, and I could see he considered his remark well and truly laid.
~ H.G.Wells in Tono-Bungay, 1909