The roots of the mandrake do look a bit like human legs,
but I don't remember any suggestion that gave it the name.
I'll go look.

From the Internet:
The derivation of the word mandragora and the English name mandrake is
still uncertain. By some, mandrake is said to be a corruption of the
Greek and Latin mandragoras, while another suggestion is that it is
compounded of the Sanscrit words Mandros, sleep, and Agora, an object or
substance, the whole meaning the "Sleep-producing drug." The Persian
names are also suggestive, thus besides merdomgia we find segken,
istereng and ebrewi ssanam. Segken means "dog-dug.", istereng may refer
to the luminosity of the root, and ebrewi ssanam "the face of an idol."

The Mystic Mandrake
by C.J.S. Thompson

Webster's 1913 Dictionary

Definition: \Man"drake\, n. [AS. mandragora, L. mandragoras, fr.
Gr. ?: cf. F. mandragore.]
1. (Bot.) A low plant ({Mandragora officinarum}) of the
Nightshade family, having a fleshy root, often forked, and
supposed to resemble a man. It was therefore supposed to
have animal life, and to cry out when pulled up. All parts
of the plant are strongly narcotic. It is found in the
Mediterranean region.

And shrieks like mandrakes, torn out of the earth,
That living mortals, hearing them, run mad. --Shak.

Note: The mandrake of Scripture was perhaps the same plant,
but proof is wanting.

2. (Bot.) The May apple ({Podophyllum peltatum}). See {May
apple} under {May}, and {Podophyllum}. [U.S.]