Green’s Dictionary of Slang

1. a general intensifier, terrifically, awfully.

1880		R. Grant Confessions of Frivolous Girl 98: ‘I’m frantically glad,’ cried Peepy, clapping her hands together.
1890		‘Rolf Boldrewood’ Colonial Reformer III 223: Frantically glad to see you, Charley, my boy.
1927		E. Wallace Squeaker (1950) 78: I know you are not frantically keen on marriage.

2. excitingly, amusingly, enjoyably.

1959

Mad mag. Sept. 41: It’s frantically cool and jivey that we’re on this kick.