Noun/Verb - Signifyin' is a form of street poetry used in the African-American community. Using a rhyme structure, it often contains a series of taunts or boasts. Soemtimes called "playin' the dozens".
As spoken by Rudy Ray Moore (aka Dolemite):
The Signifyin' Monkey
Way down in the jungle deep,
The bad ass lion stepped on the signifyin monkey's feet.
The monkey said, "Muthafucka, can't you see?
Why, you standin on my goddamn feet!"
The root of the word is "sign", as in pronouncing or interpreting "signs" of the future or important spiritual symbols. This must be related to African or Afro-American religious and spiritual practices involving the serious disucssion of divine signs and their meanings.
In a more general sense, if someone is said to be "signifyin"", it may mean that they are taking themselves to seriously by holding forth on deep portentious subjects in a self-important manner.
One might be describing pretentious intellectualizing or over-bearing preaching about matters which should make us humble by their mystery.
When asked to sign a timesheet at work, the manager said, "Sure.. in Heberew or English ?"
On which a passing co-worker commented, "There he goes signifyin' again."
Since I happen to be Jewish, I know exactly what he meant.