Dat (blank) doe is a catchphrase that can be translated to formal English as “that (blank) though.” It is used to emphasize that something is particularly awesome and is often capitalized. For example, DATBACKFLIPDOE (http://seenive.com/v/987825434643623936), DATFREEZEDOE (http://www.youtube.com/watch?v=0t_TZC0BIeA), or commonly DATASSDOE (http://www.youtube.com/watch?v=m_9rCQnfFJ4).
As the definition of “doe” in formal English is a female deer, some have used this double entendre to comedic effect (http://weknowvines.com/dat-ass-doe-vine/).
As can be seen here, the trend started in 2013 and is currently rising: