1564, from
petty (q.v.), the second element possibly from obs. Du.
focker, from Flem.
focken "to cheat," or from cognate M.E.
fugger, from
Fugger the renowned family of merchants and financiers of 15c.-16c. Augsburg. In Ger., Flem. and Du., the name became a word for "monopolist, rich man, usurer."
"A 'petty Fugger' would mean one who on a small scale practices the dishonourable devices for gain popularly attributed to great financiers; it seems possible that the phrase 'petty fogger of the law,' applied in this sense to some notorious person, may have caught the popular fancy." [O.E.D. first edition, in a rare burst of pure speculation]
However, cf.
pettifactor "legal agent who undertakes small cases" (1586), which, though attested slightly later, may be the source of this.
[pet-ee-fog, -fawg] 
