Like hippo (from hypocrite), but also with a tiny body part. A "big mouth hippo" is a person who says something is wrong one day but then goes and does that exact same thing or has already done it in the past. Calling someone a tiny hippo is also making reference to their penis or if they are a women, their ass. So big mouth, small dick. Or big mouth, small ass (or tight arse - vulgar form). It is sometimes used to refer to a gay person who talks hypocritically, normally a bottom/submissive in the relationship.
by Dysklyver December 6, 2017