John Aubrey commented in 1686: ‘To sit cross legged or with our fingers pectinated, shutt together, is accounted bad. Friends will psuade us from it. The same conceit was religiously observed by ye ancients as is observable from Pliny’, and also ‘When one has ill luck at cards, ‘tis common to say that somebody sits with his legges acrosse, and brings him ill luck’ (Aubrey, 1686: 111, 199). Most other reports, from before his time until the present day, report crossed legs as a way of ensuring good luck, especially at cards, or (in 1671) girls wanting luck in the lottery by ‘praying cross-legg'd to S. Valentine’. In two specific situations, however, crossed legs must definitely be avoided—while someone else is giving birth and, according to a note in Folk-Lore (33 (1944), 390–1), at a spiritualist seance.
Compare CROSSING FINGERS.
Opie and Tatem, 1989: 109–10.