Green’s Dictionary of Slang

extraordinary, esp. admirable, remarkable.

1812		Southey Essays (1832) I 111: During the last forty years, a tremendous change has been going on; it has affected all classes .
1885		Bulletin (Sydney) 25 Apr. 17/1: Not but the rain is badly needed, for our vegetable bills have been something tremendous lately owing to the scarcity of rain, but for my part I’d rather go without cabbages for ever than have weather like this.
1898		E. Pugh Tony Drum 23: It’s a sort o’ grating, for all the world, like a tremenjous big sewer-sink.
1909		J.W. Carr in ‘Word-List From Northwest Arkansas’ in DN III:v 405: tremenjous, adj. Tremendous. ‘His success as a public speaker was something tremenjous.’.
1933		Hecht & Fowler Great Magoo 127: I’m workin’ on the biggest, Goddamn tremenjous thing that’s happened on this little old street in years.
1958		M.A. Crane ‘Miscellany’ in AS XXXIII:3 225: Among nonhipsters, the most widespread of all hip expressions are those expressing warm approval: choice, tremendous, makin’-it, crazy, cool, way out.