Green’s Dictionary of Slang

1. the very last few weeks or days of one’s sentence.

1914		Wash. Post 11 Nov. Miscellany 3/6: ‘Wop’ is a few days or weeks yet to serve before the gate lifts and the end of the ‘bit’ has come.
1950		Goldin et al. DAUL 238/2: Wop. (P) Any fractional portion of any calendar unit of time. ‘Three (years, or months, or weeks, or days) and a wop (a fraction of a year, or a month, or a week, or a day) and I hit the pavement (will be released).’.

2. a short sentence.

1928

Rocky Mountain News (Denver) 13 Nov. in AS III:3 254: If he [i.e. a criminal] has a good mouthpiece, he may get by with a ‘wop.’.