Tharki is a Hindi word which means lascivious, pervert or lewd. Chhokra is Hindi which means a boy, a guy, and chhokro is a dialectic version of it, spoken in Rajasthan and a few other Indian states.
Hence, Tharki Chhokro means a lascivious boy, or pervert guy.
Tharki Chhokro [or Tharki Chokro] is a song in Aamir Khan starrer PK.