Verb that happens when someone gives a sip of their beverage (perferabally a canned beverage) and the person who's drinking puts their mouth all over it.
Matt- Hey there Sally, can I have a sip of your new beverage straight out of that can?
Sally- Why yes, you can.
*Matt takes a sip*
Sally- What the hell Matt why would you fish-mouth the drink like that, i don't want it now.
by mikeamil January 31, 2008
When your butthole continuously flexes during or after a Hardy poo. The action is is like a fish gasping for air.
That turd was so big that I'm fish mouthing over here.
by Ben Jiggins December 20, 2017
Eye of a penis, small and slitty
Jeremy had an STD which caused a painful sensation when he pissed through his fish mouth
by deadfrog1976 October 16, 2007
A prevalent facial defect found on SJWs and feminists where their mouths are permanently slunk down to the sides of their chin, thus giving the appearance of a retarded fish. These features are often accompanied with the standard privilege checker glasses, a huge honking nose that blows when triggered, and unfitting hair colors.
Doctor: I'm sorry to say that you have developed Fish-mouth Syndrome due to prolonged exposure to Tumblr.

Girl: PATRIARCHY!!!!!!!!!!!!!!!
by IReallyDontLikeYou May 12, 2016
Fish-mouth Syndrome is a condition that infects most of the SJW community. You can tell if someone suffers from this just by their upright-triangular-frowning mouth appearance (which also exposes the front incisors). This facial expression gives the infected person a stink face, with subtle hint of mental retardation.

Patients with FMS (abbreviation for Fish-mouth Syndrome) often have attention seeking hair styles/color, wear nonprescription glasses, and have at least one body piercing. They are easily "triggered", which prompts them to uncontrollably defecate from their mouths.

Cure: Common sense and more exposure to the real world.
Barista: Welcome to Starbucks ma'am. How can I help you?

Fish-mouth Syndrome patient: Oh my god! Did you just assume my gender? How dare you?! I refuse to be oppressed by your white cis male privilege. I will boycott all coffee from now on since you offend me so much. RAGE RAGE AGAINST PATRIARCHY! Now if you would stop harassing me, I have a pitcher of male tears to consume. Check your privilege.
by ForFuckSakeMate September 12, 2016
A girl or a guy who doesn't enjoy giving oral. They leave their mouth open and don't suck or move their tongue.

By not doing anything with their mouth or tongue, the other person is getting no enjoyment out of receiving head.
She just wasn't into the bj, I might as well just have stuck my wang in a large mouth bass, woulda felt the same. I hate when I get bad head, damn dead fish mouth girl!
by Dex Doom December 20, 2013
Used to describe when a Girl has a set of those puckered up lips and it looks like they are too small to give any head.
Damn! That girl you were talking to was fine but it looks like she's got the Baby fish mouth.
by ZZZSpidey December 22, 2006