Oh, Lesbia is a woman! The lover of many in ancient Rome. Actually, her real name was Clodia. Lesbia is just the name Catullus, the great love poet gave her so he could write about her with a secret name. That was totes the style in those day. Catullus drew Lesbia's name from the Greek poetess Sappho who lived on the island of Lesbos. Actually, I think a person from Lesbos is technically a "Lesbian". But anyways, Sappho used to hold these sort of secret girly club meetings where she taught other ladies "poetry" on the island of Lesbos. So all the Greek dudes thought (perhaps rightly so) that they were up to something else *cough*hack*cough*, so they were all like "She seems like a Lesbian, if you know what I mean!!! "
Hence, Applejack's contemporary usage of the term.