The act of logging in to MSN Messenger (or some other messenger service), opening a conversation with another person with the sole purpose of cursing them out, and then signing off before the other party has the chance to respond.
Güerito and güerita are the diminutive form of "güero" and "güera," which refer to light skinned, usually blonde people (but not always blonde). In many cases they also have blue, green, etc. colored eyes. By using "ito" the user shows some type of affection. Güero and güera also tend to be used as nicknames. In other cases the terms may refer specifically to anglos/gringos/as and/or gabachos/as.