Usually a nickname for a male who has such small genitalia that it is impossible to pleasure a female during intercourse; Inevitably, women don't even know they've been penetrated due to the size.
(n)- When someone, particularly a female, gets so pissed that she purses her lips into the smallest indication of a mouth possible, resembling the asshole of a ferret, in shape and size.
I'm not the one who took your stupid highlighter pen, so don't get the tinymouth with me.