Green’s Dictionary of Slang

1. to write for money.

1899

A. Binstead Gal’s Gossip 86: He is reduced to frogging out sermons at half-a-guinea a pair for old comrades who were more fortunate.

2. (US campus) to cheat.

1900

E.H. Babbitt ‘College Words and Phrases’ in DN II:i 37: frog, v. To cheat or deceive, especially in examination.