What is the X in X-mas?
c.1200, from Old English blostm, blostma "blossom, flower, fruit," from Proto-Germanic *blo-s- (cf. Middle Low German blosom, Dutch bloesem, German Blust), from PIE *bhlow-, extended form of *bhel- (3) "to thrive, bloom," possibly identical with *bhel- (2) "to blow, inflate, swell" (see bole). This is the native word, now largely superseded by bloom and flower.
late 14c., from Old English blostmian, from blostma "blossom, flower" (see blossom (n.)). Figurative use from late 14c. Related: Blossomed; blossoming.