Green’s Dictionary of Slang

blow one’s horn v.

[play on SE/blow v.1 (1e)/blow v.1 (2a)]
(US)

1. to speak or sing out of turn.

[US]A. Greene Glance at N.Y. II ii: lize.: And if it wasn’t for bein’ in the street, I’d sing it for you. mose.: It’s too early in de mornin’ for many folks to be out – so you’re safe. Blow your horn.
[US]A.H. Lewis Wolfville 251: I reckons Billy’s merely blowin’ his horn; bein’ sick an’ cantankerous with his game knee.

2. (also blow one’s own horn, toot one’s horn) to brag, to boast.

[US]Essays and Reviews (Princeton U.)37: From the knowledge of Castner's character, gained by careful observation, we have always considered him perfectly capable of ‘blowing his own horn ’.
[US]Gardener’s Mthly Aug. 244/2: He is averse to ‘'blowing his own horn;’ but good skill requires as much advertising before it is known as the Quack's business does.
[US]Medical Gaz. 407/2: Dr. Swinburne [...] certainly is an adept in ‘blowing his own horn.’ If all that is stated in the report is to be belived, Dr. S. would appear to be the leading surgeon of America, if not of the world.
[UK]M. Davitt Leaves from a Prison Diary I 150: Young thieves would, of course, ‘blow their own horn’ in narrating their sentences and exploits.
[US]F.P. Dunne Mr Dooley’s Philosophy 18: I think Tiddy Rosenfelt is all r-right an’ if he wants to blow his hor-rn lave him do it.
[US]W. Winchell On Broadway 15 Oct. [synd. col.] Remember when only the columnists used to toot their horns about their scoops?
[US]‘Hy Lit’ Hy Lit’s Unbelievable Dict. of Hip Words 3: When one is barking, he is bragging, blowing his horn, chest-beating about his greatness.
[US]D. Burke Street Talk 2 187: Every time I see him, he blows his horn.