Refers to “scissoring” which is when two people rub their vulvas and clitoris together as a form of non-penetrative sex. Not specific to humans, this behaviour is also observed in nature. In particular bonobo monkeys, where vulva to vulva genital rubbing is common (about once every two hours). 
If you’ve frequented sites with erotic content, you’re probably familiar with the term tribbing, an erotic act which features two people grinding their vulvas on each other.
As an act totally outside the sexual realm of men, tribbing fell under intense scrutiny, as something to be studied and objectified. But where did the term come from?
The Origins of Tribbing
As far back as ancient Greece, the words used to describe lesbians were derived from the acts they performed together. Sexuality was not an identity – it was literally what sex you had.
Lesbian sex involved grinding or rubbing together and so the term for lesbian was a derivative of the verbs that described rubbing or grinding.
Tribo = to rub; Tribas = a lesbian; Tribadism = lesbianism
S/h/q = to grind; Sahq = lesbian; Sahhaqat = to make love in a rubbing fashion
But rubbing was not the only metaphor that those early scholars used to categorise lesbians. In Ancient Greek, a tribade could be defined in two contexts – military and sexual. In the military context, the tribade was a shield, and this double meaning can be seen throughout medieval erotic literature where lesbian lovemaking is often described as ‘the banging of shields without the use of a lance’. 
The popularity of this analogy in the Middle East meant that it quickly cross-pollinated into Europe during the Crusades and appeared in Etienne de Fougères’ Livre des manières, written around 1174. 
These ladies have made up a game:
With “trutennes” they make an “eu”,
they bang coffin against coffin,
without a poker to stir up their fire.
They don’t play at jousting,
but join shield to shield without a lance.
They don’t need a pointer in their scales,
nor a handle in their mould.
Out of water they fish for turbot
and they have no need for a rod.
They don’t bother with a pestle in their mortar
nor a fulcrum in their see-saw.
They do their jousting act in couples
and go at it full tilt;
at the game of thigh-fencing
they lewdly share their expenses.
They’re not all from the same mould:
one lies still and the other makes busy,
one plays the cock and the other the hen
and each one plays her role.
This poem follows a passage which is very anti-homesexual acts, so the writer probably does not intend the audience to look upon them favourably. However, it is still amazing to be able to look back 900 years and see within a text a person like myself. I’ll have to perform this poem soon.
Edit: Very soon indeed. Here’s the above extract from Livre des manières, as performed by me this evening.
 Definition from the Urban Dictionary, edited by me.
 In fact, it’s so frequent that scholar Sahar Amer considers it one of the most identifying trademarks of the Arabic homoerotic tradition.
 The quoted words were unable to be translated.
Sahar Amer, Crossing Borders: Love Between Women in Medieval French and Arabic Literatures, (University of California Press: 2008).
Poem translated by Robert L.A. Clark. Source: The Naked Philologist, “Banging Shield and Shield Together: Lesbians in Medieval French Literature”, July 1, 2008. Viewed January 28, 2018.