Green’s Dictionary of Slang

mortar n.

1. the vagina; cits. 1719, c.1750 are double entendres.

[UK] in D’Urfey Pills to Purge Melancholy II 237: A Vision too I had of old, / That thou a Mortar wert of Gold! / Then cou’d I but the Pestle be, / [...] / Oh! how I wou’d pound my Spice in thee.
[UK] ‘The Proud Pedlar’ in Ebsworth Roxburghe Ballads (1893) VII:1 54: The mortar was your own Lady’s, but the pestle was my own.
[UK]‘Bumper Allnight. Esquire’ Honest Fellow [as 1719].
[UK]‘The Ladies Doctor’ in Secret Songster 28: When I only get into their mortar, / With my pestle I hammer away.
[UK]Farmer & Henley Sl. and Its Analogues.

2. butter.

[UK]Flash Mirror 20: Small conductor and mortar two win.

In compounds

mortar-piece (n.)

1. a prostitute.

[UK]Mercurius Democritus 24-31 May 26: Then Mourn you wanton Morter-pieces all, / This is a Signal of your total fall.
[UK]‘Peter Aretine’ Strange Newes 2: Bette. They may be as well brass’d as any Mortar-piece that was in St Jameses Fair .

2. the anus.

[UK]Laughing Mercury 25 Aug. - 8 Sept. 175: [She] let such a Bum-cracke that with the winde of her Morter-piece she blew down London-bridge.

3. a wife.

[UK]Mercurius Democritus 31 May-7 June 38: The poor contented soul his own Morter-piece comming before the Justice gave her verdict that she [i.e. a prostitute] was an honest Vertuous woman.
[UK]Mercurius Democritus 22 May 6: He would not do his druggery in the Antipodes of his own Morter-piece .