Much like a rhetorical question, a rhetorical hashtag is a hashtag used in a tweet to produce an effect or make a statement, rather than for the generally intended purpose of hashtag, which is to connect the tweet with other tweets using the same tag and make it searchable. Rhetorical hashtags are often used to inject humor and sarcasm, or even to clarify what the tweet pertains to. Occasionally they are used to make passive aggressive jabs at others. They are sometimes long and made up of many words strung together.
Some examples of tweets using rhetorical hashtags: "I'm awake. #crap" "So @thinkgeek is hiring product testers? They should totally hire me to take care of that for them. #onlyhalfkidding" "Sure, I'll let my little brother borrow my new car. #overmydeadbody #worstdriverever"
Sharon really goes overboard with the rhetorical hashtags. She can hardly tweet without adding something like #holyhellwhyisthishappeningtomeagain. I wish she'd shut up and quit whining.