Wiktionary's entry for 冷奴 says:
From 冷【ひ】や (hiya, “cold (in compounds)”) + 奴 【やっこ】 (yakko, “male servant”); the significance of the latter is debated.
Which fits Wiktionary's other entry for 奴:
奴 (hiragana やっこ, rōmaji yakko)
servant, valet, footman
Wiktionary's entry for 冷奴 also notes:
The literal interpretation of the characters as “cool guy” has seen this word being used on T-shirts (and so on) for stylistic and humorous effects.
So is it “cold slave” or “cool guy” and what is the 奴 doing here in 冷奴? I realize that Wiktionary claims its "significance ... is debated," but what things are being debated I'm not even sure.