A dumbo drop is similar to a rusty trombone. It is where you are receiving a handjob while the person is licking or eating out your asshole. However, what makes it a dumbo drop is the act of defecating while they are doing this.
Man, this girl last night was nasty. While she was giving me a rusty trombone I had to shit so I decided... fuck it... I'll just dumbo drop her.
While reverse titty-fucking a girl, reach your arms around and pull her ears. While her ears are fully extended outward, you presume to take a dump in her mouth.
"Dude! She totally didn't know the dumbo drop was coming!"