Middle English word blissen comes from Middle Dutch be- (A verb prefix with a variety of meanings.), Middle Dutch lesschen, Middle Dutch lusschen, Middle Dutch bleschen, Proto-Germanic *blīþisjōną (To be good, be kind, be joyful.)

Detailed word origin of blissen

Dictionary entry	Language	Definition
be-	Middle Dutch (dum)	A verb prefix with a variety of meanings.
lesschen	Middle Dutch (dum)	To calm, to ease. To extinguish. To go out (of fire). To lessen, to reduce. To quench.
lusschen	Middle Dutch (dum)
bleschen	Middle Dutch (dum)
*blīþisjōną	Proto-Germanic (gem-pro)	To be good, be kind, be joyful.
blissian	Old English (ang)	(intransitive) to rejoice; be glad. (transitive) to gladden; make happy; cause (someone) to rejoice; delight.
blisscen	Middle Dutch (dum)
blesshen	Middle English (enm)

Words with the same origin as blissen