Source : Webster's Revised Unabridged Dictionary (1913)

Overstrain \O`ver*strain"\, v. t.
   To stretch or strain too much; as to overstrain one's nerves.
   --Ayliffe.

Overstrain \O`ver*strain"\, v. i. [imp. & p. p. {Overstrained};
   p. pr. & vb. n. {Overstraining}.]
   To strain one's self to excess. --Dryden.

Source : WordNet®

overstrain
     n : too much strain
     v : strain excessively; "He overextended himself when he
         accepted the additional assignment" [syn: {overextend}]