Green’s Dictionary of Slang

1. (US Und.) to release from custody.

1924

G. Henderson Keys to Crookdom 405: Flagged. Released from custody.

2. (US Und.) to arrest.

1927		‘Jargon of the Und.’ in DN V 446: Flag, (2) To be arrested.
1964		‘Return of Honky-Tonk Bud’ in D. Wepman et al. Life (1976) 62: They got me up tight, and you know that ain’t right. / In fact they even flagged me wrong.
1970		G. Cain Blueschild Baby 55: Her son came by [...] don’t know nothing bout no dope. They flagged him in the hall and put some stuff in his pocket. Breeze her old man got busted too.

3. (US campus, also fly a flag) to fail a test or examination; thus to get a grade F in an examination.

1959		Maier College Terms 3: Flag a test – Flunk a test [HDAS].
1963		Dundes & Schonhorn ‘Kansas University Sl.: A New Generation’ in AS XXXVIII:3 168: To fail to pass an examination: flag.
1965		Wisconsin State Jrnl 17 Jan 1-2: Getting an ‘A’ on a test is ‘aceing’ it or ‘hooking’ it. Getting an ‘F’ or failing is called ‘flagging’.
1966		Current Sl. I:1 2/2: Flying a flag Failing a test or course.
1968		Current Sl. III:1 6: Flag, v. To fail.
1977		Eble Campus Sl. Fall 3: flag – to fail a test or course.
1987		Eble Campus Sl. Oct.
1996		Eble Sl. and Sociability 34: flag ‘make the grade F’ (‘I’m afraid I flagged that test’).

4. (US campus) to fail to attend a class.

1992

D. Burke Street Talk 2 17: She’s gonna fail if she keeps flagging class all the time.

5. (N.Z., also flag away) to give something up, to abandon.

2003

McGill Reed Dict. of N.Z. Sl. 78: flag/flag away Give something up, lose patience or confidence.