It's the present participle of the verb "Reminisce", which is a synonym for the verb "Remember".
It is a very classy verb used by various poets and
people of higher status.
It was also made famous by
rapper Tupac Shakur's "Pour out a little liquor" song which made various Afro-American L.A. gangsters use it while smoking
weed and drinking liquor with their bitches in the club.
Person
1 : Yo wazzup man! Watchu u doin here sHHmokin' that
weed all alone?
Person 2 : Imma just reminiscing mah good old days aZ a straight out G!
Person
1 : Mind if join mah homie?
Person 2 : aiight foo!