What does 고양이소 mean?

Answer: a hypocrite; a wolf in sheep's clothing

고양이소 literally means "the characteristics of a cat," which Koreans apparently believe is a selfish pretender, pretending to be nice and innocent when it really is not. 고양이 is, of course, the pure Korean word for cat, and 소 is the same 소 in 소질 (素質), which is a Sino-Korean word that translates as characteristic, nature, or temperament, so 고양이소 could also literally translate as the nature of a cat or the temperament of a cat. 고양이소 seems to come from the Chinese 猫素 (묘소). 猫 (묘) is the Chinese character for cat, and 素 (소), here again, means characteristic or nature.

고양이소 is in my medium-sized Korean-English dictionary, but I am not sure how often Koreans use it, so to avoid having people think you are talking about some kind of cow (소), you might consider saying 고양이 소질 instead of simply 고양이소.