mandrake, n. — Green’s Dictionary of Slang

mandrake n.

[SE mandrake, ‘Any plant of the genus Mandragora…characterized by very short stems, thick, fleshy, often forked, roots, and fetid lance-shaped leaves. The mandrake is poisonous, having emetic and narcotic properties, and was formerly used medicinally. The forked root is thought to resemble the human form, and was fabled to utter a deadly shriek when plucked up from the ground’ (OED)]

1. a dildo.

1655

Mercurius Fumigosus 38 14–28 Feb. 303: They walk out hand in hand like two disconsolate Virgins to seek Mandrakes to help them make perfect what their lost Sweethearts have left behinde, their concupiscense being so predominant in the House of Venus.

2. (late 18C) the penis.

1784

Rambler’s Mag. June 227/2: Besides the common name of Arbor Vitae [...] others would have called it the Mandrake of Leah persuaded it is the same whose juice made the before-barren Rachel a joyful mother.

3. a sodomite, a male homosexual.

[1604		Dekker Honest Whore Pt 1 I ii : Gods my life, hee’s a very mandrake, or else (God blesse vs) one a these whiblins, and thats woorse, and then all the children that he gets lawfully of your body sister, are bastards by a statute].
c.1880		R. Bunting (compiler) San Diego CA (2001) US Army Sl. 1870s–1880s n.p.: Mandrake A homosexual.