The Collaborative International Dictionary of English v.0.48:
Mar \Mar\, v. t. [imp. & p. p. Marred (m[aum]rd); p. pr. & vb.
n. Marring.] [OE. marren, merren, AS. merran, myrran (in
comp.), to obstruct, impede, dissipate; akin to OS. merrian,
OHG. marrjan, merran; cf. D. marren, meeren, to moor a ship,
Icel. merja to bruise, crush, and Goth. marzjan to offend.
Cf. Moor, v.]
1. To make defective; to do injury to, esp. by cutting off or
defacing a part; to impair; to disfigure; to deface.
I pray you mar no more trees with wiring love songs
in their barks. --Shak.
But mirth is marred, and the good cheer is lost.
Ire, envy, and despair
Which marred all his borrowed visage. --Milton.
2. To spoil; to ruin. "It makes us, or it mars us." "Striving
to mend, to mar the subject." --Shak.