Green’s Dictionary of Slang

lappy adj.2

[backsl. lappy: pally adj. (1)]

(UK und.) for friends only, private.

Police Chron. (London) 3 Sept. 5/2: My tramp acquaintance could make nothing of the sign; he described it in back-slang as a ‘lappy’ (pally) mark; only a private communication from one vagrant to a chum, and not intended for the enlightenment of the general road community.