Green’s Dictionary of Slang

(UK Und.) a padlock.

1865		Leaves from Diary of Celebrated Burglar 88/1: Upon reaching the place Joe ‘turned off’ the ‘paddy’.
1904		Number 1500 Life In Sing Sing 258: He sprung the paddy with a screw. He opened the lock with a key.
1924		G. Henderson Keys to Crookdom 413: Paddy. A padlock.
1935		Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 8: Paddy: Padlock.
1949		Monteleone Criminal Sl. (rev. edn).
1959		‘Screwsman’s Lament’ Encounter n.d. in Norman Norman’s London (1969) 67: We went round to a lockup, to nick ourselves a drag, / We took the paddy off the door, the rest was in the bag.