This is the largest "sigh" a human can produce. Usually associated with disdain or disgust , misery and woe, or a women on her period. It comes from the comparison of the large exhaling released by a hippopotamus during the breathing process.

Internet persona(s) who happens to follow you and you happen to follow them back and maintain communication with and have a mutual liking for eachother whether it be friendship wise, etc. (Mutuals if more than one mutual)