What is the X in X-mas?
c.1300, perhaps via Old French blestre "blister, lump, bump," from a Scandinavian source (cf. Old Norse blastr "a blowing," dative blæstri "swelling"), or from Middle Dutch blyster "swelling;" perhaps from PIE *bhlei- "to blow, swell," extension of root *bhel- (2) "to blow, inflate, swell;" see bole.
"to become covered in blisters," late 15c.; "to raise blisters on," 1540s, from blister (n.). Related: Blistered; blistering.
blister blis·ter (blĭs'tər)
A local swelling of the skin that contains watery fluid and is caused by burning, infection, or irritation.