Green’s Dictionary of Slang

(US campus) to vomit.

1959		L.P. Boone ‘Gator Sl.’ AS XXXIV:2 156: As a result of this state [i.e. drunkenness], they may flash (vomit).
1964		Current Sl. (1967) I:4.
1965		Poston & Stillman ‘Notes on Campus Vocab.’ in AS XL:3 194: flash, v. To vomit.
1968		‘Sl. of Watts’ in Current Sl. III:2.
1970		Current Sl. V:1.