Green’s Dictionary of Slang

1. a tuba or saxophone.

1919		G.W. Small Story of the 47th 119: Joe extracted his ‘foghorn’ [i.e. saxophone] from its unshapely box [HDAS].
1934		Charleston Dly Mail (WV) 31 July 6/8: Musicians have slang terms for every instrument [...] Fog horn – bass saxophone.

2. the nose.

1942		Berrey & Van den Bark Amer. Thes. of Sl. 121.69: Nose [...] foghorn.
1968		in DARE.

3. one who talks too loudly; thus the mouth.

1886		Clarkesville Wkly Chron. (TN) 16 Oct. 1/3: Fog-horn Pete drove a pretty one into left field.
1907		W.M. Raine Wyoming (1908) 105: Tame that fog-horn, son.
1931		B. Lubbock Bully Hayes 2: Such laughter indeed as had gained him his nickname of ‘Foghorn’.
1936		R.F. Adams Cowboy Lingo 176: ‘Foghorn’ Lacey was dubbed thus because he talked in such a loud tone of voice.
1964		N.B. Harvey Any Old Dollars, Mister? 153: ‘Hold your tongue, foghorn,’ Dad said to me.
1968		in DARE.