Mystic Meg, n. — Green’s Dictionary of Slang

Mystic Meg n.

[rhy. sl.; ult. Mystic Meg, a UK astrologist, made famous in the 1990s by her appearances on the National Lottery TV show]

1. usu. in pl., the human legs.

2001		Simon Arseweb.com Newsreel 15 May 🌐 David Seaman’s problem, apparently, is that he doesn’t keep his legs closed. [...] maybe it’s still worth keeping the ol’ Mystic Megs together Dave, eh?
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.

2. the penis, as in ‘third leg’.

2003