Green’s Dictionary of Slang

1. the penis.

1631

J. Donne ‘To His Mistris Going to Bed’ in Gardner (2000) 15: Then where my hand is set, my seal shall be. / Full nakedness! all joys aru due to thee .

2. the vagina.

1596		Chapman Blind Beggar of Alexandria vii: Now, Madam. Countess, do you make account To take up husbands by your countess-ship? Have you the broad seal for it; are you so high, And stoop to one so low as is my husband?
1661		‘Maid a Bathing’ in Ebsworth Choyce Drollery (1875) 230: Her legs she opened wide, My eyes I let down steal, Untill that I espy’d Dame natures privy Seal.

3. in pl., the testicles.

1781		Belle’s Stratagem 3: Maas Ogul’s fille de chambre, Riggeltail, had a very grand design to ravish me of [...] my vatch, mi Lord, and de two stones dat hang to it — de seals.
1890–1904		Farmer & Henley Sl. and Its Analogues.