It is probably of Dutch ancestry, and David Parlett suggests that it played a role with the association of the name Jack with the card rank that was originally known only as the knave.
Next, naked, gas-masked people (the frightened ones) are seen running about on all fours and hiding from The Blitz.
Tom Baker detested the character of K-9, but luckily got on very well with Leeson (when Leeson appeared on Tom Baker's edition of This Is Your Life in 2000, Leeson entered on all fours).