Green’s Dictionary of Slang

1. used by a man on seeing a passing attractive young woman; or vice versa [? an imitation of the howl of a lovesick dog].

1927		H.C. Witwer Classics in Sl. 49: Ethel is walkin’ around the place checkin’ off books and whilst she is poor, she is a up-to-date dresser and you know that high necks and long skirts is barred now and — Woof!
1977		Eble Campus Sl. Fall 6: woof! – an exclamation of approval of a male’s looks.
1992		Eble Campus Sl. Apr.

2. a non-sexual excl. of appreciation.

1944		T. Thursday ‘Twin Lose or Draw’ in Popular Sports Spring 🌐 Me, I was rooting for him, but the way he did it — woof!
1948		T. Thursday ‘Movie Stuff’ in Detective Story Apr. 🌐 ‘Woof!’ exclaimed Kreuger. ‘I just bought a suit in his joint last week.’.