a small black girl character in the novel Uncle Tom's Cabin (1851-52) by Harriet Beecher Stowe. When somebody asks her whether she knows who made her (that is, whether she has heard of God), she replies "I expect I grow'd" (=grew). People say something "just grew, like Topsy" when they are talking about something whose real origin is not known or about something that has gradually become very large.
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.