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.