What does the word rub mean?

It appears that because the English word tribade originally referred to any homosexual woman, tribadism was originally essential a synonym for lesbianism. According to the OED, this is still its primary meaning, although the examples make it clear that from 1965 on, the term is used to describe vulva-to-vulva stimulation specifically. This specific usage was probably influenced by a reinterpretation of the literal meaning of tribade. The term is thought to ultimately derive from the ancient Greek word "to rub" tribein τρίβειν. This understanding probably influenced how the term came to specify female-female genital sex, as mutual genital "rubbing". The English verb to trib derives from an abbreviation of tribadism.