Green’s Dictionary of Slang

to break open with a crowbar (cf. jimmy v.¹ (1)).

1872		Sportsman (London) ‘Notes on News’ 18 Jan. 2/1: Evry street Aran who has srtolen a ‘wipe’, every burglar who has ‘jemmied’ his way into a private dwelling [etc].
1933		‘Goat’ Laven Rough Stuff 20: I [...] jemmied open a window which was not protected by the alarm.
1996		A. Close Official and Doubtful 294: Now Max is jemmying up the floorboards, uncovering another body.
2000		Indep. on Sun. 16 Jan. 28: Jemmied open a few windows?
2001		K. Sampson Outlaws (ms.) 8: We’d [...] jemmy the back doors open.
2004		P. Howard PS, I Scored the Bridesmaids 59: Jemmied open my filing cabinet.
2009		P. Temple Truth 174: Two new padlocks secured the steel front door. An attempt had been made to jemmy it.