Green’s Dictionary of Slang

1. (US campus) a freshman [Ger. Frosch, a frog, used of freshmen].

[1856		B.H. Hall College Words (rev. edn) 226: frog. In Germany, a student while in the gymnasium, and before entering the university is called a Frosch, – a frog].
1946		F. Eikel Jr ‘An Aggie Vocab. of Sl.’ AS XXI:1 32/1: frog, n. A first-semester freshman.
1949		Monteleone Criminal Sl. (rev. edn).

2. (US campus) a grade of F [initial letter].

1979

(con. 1964–8) oral testimony in Lighter HDAS I 368/1: When I was in college an A was an ace, a B was a bang, a C was a cat or a hook, a D was a dog, an F was a frog. B+ was a bang and a half.