Also pronounced and accordingly spelled "jimmy".
OED:
jimmy, n.1 and v.
Etymology: Dialect and colloquial pronunciation of jemmy n.
A. n.1
= jemmy n., occurring in most of the senses of that word, and in
numerous other slang, dialectal, and local uses, for which see
Dictionaries and Glossaries dealing with such uses.
** Now the more usual form in the U.S.
1848 G. W. M. Reynolds Myst. London IV. cxcv. 369/1, I have got
my own clasp-knife..and a small jimmey.
1854 Alta California (San Francisco) 23 Feb. 2/2, Officer
Powers,..upon examining the lock, found it had been broken open
with a ‘jimmy’.
1904...
1973 ‘E. McBain’ Let's hear It xiii. 194 The patrolman..was
examining a door and jamb for jimmy marks.
....
1973 P. B. Austin tr. M. Sjöwall & P. Wahlöö Locked Room xxiii.
186 [He] had brought with him every thinkable jimmy and other
tool for opening the door.
--
Peter Duncanson, UK
(in alt.usage.english)