Green’s Dictionary of Slang

(US teen) to make a farting noise with the lips.

1933		J.T. Farrell Gas-house McGinty 35: Louis lip-farted. That for you, Mushmouth!
1936		J.T. Farrell World I Never Made 133: ‘Artie Lenehan, who always makes trouble and does this all the time,’ Danny said, lip-farting to show his grandmother what Artie always did.
1958		T. Berger Crazy in Berlin 107: The Russian scratched an elusive cap-a-pie itch, lip-farted at his own image in the mirror.
1962		T. Berger Reinhart in Love (1963) 312: C. Roy lip-farted and took his cap off.