Tagdef.com is a service to find out what Twitter hashtags mean and add your own entries to their database quickly and simply. It also shows popular hashtags, the top ones of the week, and the most popular of all time. "#tbt", which we often see in the context of 'Throwback Thursday', could also be used for 'truth be told' or 'turning back time', according to site contributors. I tested out their random hashtag function and got #lbv - Laughing but very serious.
For those who want to contribute additional hashtag definitions and participate in building the site, you can log in in a users, identified by your Twitter handle, and get hat tips for being frequent contributors. TagDef also provides an open API that developers can use to add hashtag lookup functionality to their apps.