To retweet complimentary things others say about you so everyone knows how awesome you are without actually having to say it yourself.

When you eat dirt for the first time and leave your family to build a shack in the woods

A tweet that pretends to show humility but which is actually bragging about something that happened to you. Similar to a humblebrag.

The key to a humbletweet is to pretend you don't understand why you are getting all this attention while also making a blatant attention grab in your tweet.