What is the X in X-mas?
late 14c., "have a strong effect on the mind or heart," from Latin impressus, past participle of imprimere "press into or upon, stamp," from assimilated form of in- "into, in, on, upon" (see in- (2)) + premere "to press" (see press (v.1)). Literal sense of "to apply with pressure, make a permanent image in, indent, imprint" is from early 15c. in English. Sense of "to levy for military service" is from 1590s, a meaning more from press (v.2). Related: Impressed; impressing.
"act of impressing," also "characteristic mark," 1590s, from impress (v.).